How do I know a stone is what the listing says it is?

Ask for the report, and read which laboratory issued it. A diamond over a certain size should come with a grading report naming the lab — GIA, IGI and AGI among them — and lab-grown stones must be marked as laboratory-grown on the report itself. Grades are not interchangeable between labs, so "VS1" from one issuer is not a promise of "VS1" from another; a report is only as informative as the standards behind it. For coloured stones, ask specifically about treatment: heat, irradiation, HPHT and fracture filling are all common and all disclosable. For anything sold without a report, the listing copy is the only claim you have — which is a reason to check the return window before you check the photographs.