The slots milled in a fake flash hider won't go all the way through.
The obvious way to tell if a sound suppressor is real is to shoot it If it comes through a dealer, a real suppressor will have to be legal, and therefore require NFA paperwork. In a private sale, the suppressor had better be fake, or you're going to federal pmita prison.
The muzzle end of a flash hider has a much wider opening than the diameter of the bullet. For a muzzle brake/non flash-hider, it's usually very close to the same diameter as the bullet.