Unfortunately lab-created fake gemstones are now so good that it is difficult to know if a stone is genuine just by looking, even if you are a serious jewelry hobbyist.
Today even well-experienced jewelry professionals rely upon electronic instrumentation to weed out Moisssanite and other artificial diamonds from the real thing.
If you are interested in a loose diamond or a pre-owned ring take the precaution of getting an opinion from an independent jeweler with the right instruments. Most often you won't be charged an appraisal fee for an informal check.
On significant purchases, however, a proper Gemological Institute of America appraisal is very cheap considering the peace of mind it will bring.