Easiest way to spot the newer, "G1 original" boxed knockoffs of Grimlock are to look at the box before purchase, if possible. It's the easiest one to catch because of a glaring typo right there on the front. Instead of "Dinobot Commander" it says "Dinobot Bommander", LOL. To the best of my knowledge this still hasn't been corrected by the notorious bootlegger that is putting these and other G1 complete/sealed box knockoffs out.
Loose and open is far more difficult, since these newer KOs are fairly good copies both quality wise and accuracy wise, though there are some tells you can look for. You can find all that info at various places on the web.
Chances are though, if it's a "Complete in sealed box" or "new in box" G1 Grimlock, it's a knockoff. I have a legit boxed/complete G1 Grimlock all in near mint/mint condition, another loose legit complete G1, an original bootleg G1 that is extremely good quality (major difference outside of the stickers was the red torso piece is blue), and a G2 legit blue Grimlock.