Two of the main things, of spotting counterfeits, are poor quality and misspellings of brand name, other key info, etc.
Specifics depend on what type of product it is. I think with figures, the quality of the eyes can be a huge giveaway if its legit or not. Bootlegs tend to be of cheaper material, crappy paint jobs, and so on.
But there's some grey areas like with asian dvds that are "supposedly" legit releases in their respective countries... The ones that have usually have english & chinese subtitles with japanese audio but are of iffy quality. I'm still not 100% sure on these (whether they are legit or not)...
That aside, Dealer's Hall does a very good job of keeping counterfeits out. And swap meet used to check for counterfeits but I don't know if they do anymore. But if you do spot something, let staff know. Also, be aware that Dealer's Hall also has a bunch of professional artists that sell there and it is largely obvious that their product isn't official.