I see a lot of questions here about if a certain piece is really LEGO or not and this made me wonder if there is any guideline to follow to identify real LEGO piece from the duplicate or fake one?
I checked the site and found this question but it's not what I am asking for.
The most obvious sign is the LEGO mark present on each stud*. I have yet to learn of any single copycat company that would go through the legal and technical trouble required to mold these onto every stud of every piece in a consistent fashion.
If the piece has no studs to check or the markings are present and you still have doubts you can take a suitably strong light and magnifying lens and look for a trademark or a 4-5 digit part ID. This on the right side is a very obviously visible example, others might be much smaller and better hidden. A missing ID would obviously indicate a fake piece. But even if it is present you can cross-check the existing LEGO piece databases to see if the part ID does indeed represent the part you have.
Should a bootleg company go to the lengths to correctly add all such markings, you can still take a known genuine part of the same design and check for any differences in dimensions. LEGO has notoriously tight tolerances when it comes to precise size. You can check the material as well, fakes could have different weight, sheen, flexibility, clutch power or could feel different to the touch.
And in the unthinkable case when a competitor manages to get all markings, dimension and material properties right, can evade all inherent lawsuits and their prices are not egregiously high then they maybe deserve to be used as "real" LEGO pieces :)
But only then ;)
Sidenote: Old LEGO jewels (diamonds) don't have any ID or trademark on them, the newer ones do have both though.
*: On 2020-08-30 BrickFinder published an article about chinese fake LEGO bricks being found "in the wild" with almost perfect LEGO lettering on their studs. These have nearly imperceptable differences in the lettering, the gloss and colour of the bricks and their dimensions.
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 waist 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.
