system
A chat on subreddit /r/NoStupidQuestions
human
How would I verify that number without e.g. phoning a number on the card itself?
gpt
You're not going to be able to independently verify the number, but you can verify that it's the right type of federal id along with the badge.  The good identity verification information is going to be digital keys like those used on the internet for secure connections.  Then inform the special agent that you would like to retain counsel before answering any questions, the real FBI will at that point stop questioning you (since anything more will potentially harm their case while a fake might keep going).