These are some basic checks you can carry out to find out a little bit about the nation.
Start on the nation's home page.
What's their population?
A low population indicates they're a new nation. Either they're a new player, or a new puppet. Unless you sent out recruitment telegrams, new players aren't likely to have picked your region out of the over 20,000 others as the best one to be in. So they're probably someone's puppet. If a player thought your region was cool enough to move a puppet to, surely they would say hello? Why are they sitting there silently? That's got to count as suspicious behaviour.
A high population indicates an old nation. So you'd expect them to have a history of some kind of activity. If they don't have a history of activity, then that's suspicious. Genuine players will be doing at least one of the following activities: answering issues, collecting cards, writing dispatches, posting on RMBs, or posting on Forums.
Check where they've been
NS History can be used to show which regions a nation has been in, and if they were a WA member. Remember, this is a series of weekly snapshots, so the times inbetween aren't recorded.
View their forum posts
At the bottom of the nation's page, there might be a button "View Forum posts".
If it's not there, it must be because the nation has never posted on NS forums.
If it is there, click it. You may get a message that the nation has zero forum posts.
Or you may find a list of their posts. Read some of their posts to get an idea of who they are.
Look at the National Happenings
The bottom of their nation screen shows the ten most recent National Happenings. If all ten are within the past five days, then click on the "More..." link beneath them. The "More..." link will show you all of their Happenings in the past five days.
Can you see the region they moved from? That's a place you could go to investigate further.
Did they change flag and custom fields before moving to your region? Why would they do that?
Have they been answering issues? If so, hover your mouse over the time-stamps to see the exact minute & second that the issue was answered. Is the gap between answers long enough for the player to actually think about their answer? or were they just selecting randomly at a fast pace?
If they're selecting randomly at a fast pace, either they're just doing issues to get Card packs, or possibly they're a raider trying to look like an issues player.
If they're currently a WA member, can you see them joining the WA on the Happenings? Raiders generally move their WA between their puppets fairly frequently. If the nation has been in the WA for a long time, that's a reassuring sign. So if you don't see them joining the WA, what is the oldest event you can see? They must have joined the WA before that.
View their RMB posts
Go to any RMB board (not the Region page, the actual board). Underneath the region name is a "Search" link. Click on it. If the screen doesn't already show the "Author" and "Region" boxes, click on "Advanced".
Delete the text in the "Region" box. Put the Nation's name in the "Author" box (maybe copy & paste, to ensure you have the right spelling). Click "Search Board". Because the Keywords and Region aren't specified, this should return every RMB post (that hasn't been suppressed) the nation has made in any region that still exists.
If there are no RMB posts, why not? That's believable for a new nation, but kind of weird for older nations.
If there are posts, take a read, and find out a bit more about what they're like.
When somebody posts on an embassy RMB, the post shows the region the nation was in at the time. If the post was on their own RMB, it doesn't show the name of the region.
You can still find out the region though.
Click on the time-stamp for the post. That takes you to a page that shows the post in isolation. Underneath the post is a link "Context". Click it. This takes you to the RMB where the post was made. Scroll to the top of the screen to find out which region the nation was in when the post was made.
Reading the posts around the Nation's own post can be good too. You can start to get an idea of how the player interacted with others. Maybe they reveal the names of some of their puppets in some posts.
The Trend is your Friend
On their Nation page, click the "Trend" icon (the one that looks like a graph). The first chart it shows you is the "Civil Rights, Economy, & Political Freedom" one. The chart should show data from when the region was founded to date.
When a nation answers an issue, it will affect their scores. So this chart gives you an idea as to how often a nation answers issues.
Earlier, you may have seen them answering issues on the "Happenings" section of their nation screen. If there are long flat lines up until just a couple of days ago, the nation clearly hasn't been answering issues. Why not? Why did they start now? Is this a raider ploy to pretend to you that they're an issues player?
Look at their Population chart. If it grows in a straight line, then they've never CTEd. If there are flat parts to the line, then there are periods when the nation CTEd. Being CTEd is a good reason for not having answered issues during that period.
Look at their Residency chart. A long period of growth shows they stayed in a region for a long time. Zig-zags shows that they were in and out of regions frequently. Unfortunately you can't see which regions. If they made RMB posts (see above), you might be able to work out which regions they were in and when.
Weird dips are periods where the nation CTEd. (I can't remember what flat bits represent - I'll need to check Amerion's NS guide!)
Look at their Influence chart. That should tie in with their Residency chart, except it will flatline everytime they haven't logged in for 7 days. Any prolonged period of endorsements, the line will be steeper too. Where were they then? Were they piling in a founderless region?
Look at their World Assembly Endorsements chart. Frequent spikes might suggest a tag raider - how do those dates tie with their residency? Where were they during longer periods of endorsement? (check their RMB posts). Were they piling?
Have they bothered to write any Factbooks or Dispatches? The more effort they've gone to, the more likely they're genuine.
Is there a Card of the nation? Take a look, and see what the flag, motto, and region were, on the date that the card was made. If the card is in any collections, take a look. Are they obviously one of a number of themed puppets? Does anyone hold multiple copies of the card?
If the nation did rapid-fire issue answering, then they were either card farming, or trying to look like an issues player. Take a look at their cards to try and get a better idea.
Do they have a collection? Or does their entire deck look like a themed collection?
Do they have a large amount of bank?
Look at their trades. Look at their Buys. Were they gifted cards? Who by? It could be their puppets gifting cards to the main nation. Look at their sells. Again, what was gifted to whom?
Then check out those nations...
Talk to them. Talk to their old region mates
If you're still not sure about them, you can ask them questions. Like what are their other puppets? Which of their nations has the WA membership? You could TG ROs of their old regions too. Ask them what they know about them. Ask if they were a WA member.
If the nation tells you the names of their puppets, and you're still in two minds, don't take their word for it. Telegram the puppets asking for confirmation that they're the same player.
Forums & Discord
I usually stay gameside, so this is getting beyond my area of expertise. It's possible for players to have multiple Discord IDs, but not as easy as having multiple nations. If you know the player's Discord ID, join the Discord servers of some of the larger regions, and known raider regions. You can then check which servers you have in common.
There are no answers, only questions. Unless the nation takes the delegacy, banjects you and destroys your region, you will never know for sure that they were a raider. All you can do is weigh up the probabilities, and judge whether you're willing to take the risk.
You'd probably die of boredom if you ran through all these checks everytime you encountered a new nation. These are all just suggestions. Do what you need to do to feel comfortable enough to accept or reject a new nation. Then get on with the fun stuff. That's what we're here for after all.