When I buy LEGO in bulk, I often find that there are at least a few clone brand bricks mixed in with them. What is the quickest way to weed out these non-LEGO parts?
My puny brain can only come up with one method: One Piece At A Time.
Check for the LEGO logo somewhere. Sometimes it's hard to find. On the Brick, Round 1 x 1 with Fins it's on the bottom of one of the fins.
Seems that most of the time there are minor, yet obvious, mold differences. Some brands, like Kre-O have an indent on one or more studs.
I have recently sorted some headlight bricks and 1x1 bricks with one stud on the side and found that there are many variations on those, including whether or not the stud has a through hole, or whether or not the 'back' of the brick is solid or open.
Often enough, the plastic will simply feel wrong.