I would almost completely ignore follower count, it’s more of a red flag than anything. Blue checkmarks are similarly useless.
If you are enticed by the supposed reach, perhaps the quickest first pass at validation would be to review the nature of interactions in comments and cross-check other networks they participate in. It may help to pick someone popular in your field whose reputation you would not question, and use their account as a reference point for comparison.
For example, I recently saw an account with 260k IG followers and blue checkmark whose posts average 100+ mostly generic short comments. I compared them to Andrew Kramer, who despite having 90k followers receives as many replies on posts, and those replies are longer and contain substance. Both are within video making industry.
This is not to imply that the 260k guy or their followers are necessarily bots, but their audience is largely disengaged and may be unlikely to convert into paying customers. (Personally I don’t hesitate to use blocking on inauthentic-looking accounts to avoid becoming a node in a fake network—let bots follow other bots.)
In my opinion, “valid” follower counts rarely a few thousands. If it’s much more than that, the person either purchased followers and won’t get you any leads, or (if their name is actually well-known) isn’t going to be interested in promoting products for monetary gains.