Referring to magnesite and various imitators. There is no easy to explain set of guidelines. See, touch, handle as many examples as possible over a long period of time and it becomes easier for sure.

For antique Tibetan material, look for rich deep color and all sorts of variations that don't happen with newer stabilized material. Look for wear at the ends and holes from being worn and used for generations.

Feel the weight - the cheap imitations are generally a lot lighter.

Magnesite often has a weird contrived-looking matrix like pattern, and those patterns generally appear suspiciously brownish.

And along the way you will encounter plenty of "knowledgeable" vendors who will assure you that their product is genuine. Every time I look I can find stuff masquerading as turquoise, from plastic block, to dyed howlite and magnesite. Searching this site will yield many past conversations.

Not always possible, of course, but breaking a bead is always literally revealing.

Super-lapidary Danny Lopacki participates here, perhaps he can chime in....I have only "scratched the surface"!