I feel asha stones are very realistic. The asha OEC I had looked off to me due to the cutting (long story, but has to do with the "perfect" symmetry of the constrast zones in the stones) so if someone was wearing it, I would assume it's a newly cut OEC (not antique) however I would not question that it was real.
I think the sims have gotten so good that anyone would be hard pressed to tell that it's fake without testing equipment (you should also check out Wink Jones' hand cut CZ at freecz.com, also fantastic and very inexpensive!) not to mention that they now come in a range of diamond-equivalent colors so you're not stuck with department store, plasticky D, IF which is often a giveaway. But if you could get an H color cubic? Or an L? Cut to the same standards as a diamond, available in a variety of cuts that average people don't know cz's come in (Flanders cut, etc.)