How can one tell the difference between a manmade and natural peridot?

Answer 1

Man-made gemstones that replicate natural ones (i.e. have the same composition and structure) are referred to as synthetics (synthetic ruby, synthetic emerald, etc.), whereas materials that do not have a natural counterpart are called simulants. Some simulants bear commercial names that reflect their real chemical composition (e.g., CZ for cubic zirconia), others are simply “bell-ringers” bound to catch your ear (fabulite, tanzanion, etc.).

Answer 2

There aren't many good imitatons out there for peridot, but I think synthetic forsterite may be used for peridot imitation. Optic characters (especially under polarizing light), refractive indices and specific gravities usually give the fakes away.

Answer 3

one quick way to tell is if you can see the whole gem. another is by the cut and impurities.