I can usually just tell by the color / sheen of the bricks in question, LEGO puts a lot of effort into producing consistency in their parts.
zovits mentioned a lot of good points, but there is one thing I'd add that I have noticed as of late while sorting out bulk lots of bricks: Some companies that are truly attempting to make counterfeit lego (as opposed to mega-blocs who are just compatible) have gotten close to the right shapes and colors, but are still falling short when it comes to the ABS recipe. I have found that 'FLEGO' (my term for fake LEGO) is more brittle and shatters when subject to forces that LEGO has no problem with.
For example, I recently had a mini-fig torso that I tried to pull the arms off of, and both completely shattered. Real LEGO mini-figures you can pull the arms off of as much as you want, with little effect on the parts. Older torsos will sometimes develop a waste to armpit crack, I suspect this is more a result of age than excessive force.
I suspect that their ABS formula is producing harder, and more brittle bricks that will break rather than bend. I found several bricks in the same lot that were completely shattered, with rather sharp edges. This is something I have never seen in real LEGO bricks.