Professional jewelers can tell the difference using specialized testing equipment. The equipment that distinguishes between a diamond and a cubic zirconia will not identify a lab diamond.
Real diamond vendors say that the synthetic diamonds are “flashier,” as if this were a bad thing. (Long ago, diamonds themselves were considered too gaudy for ladies to wear; they were reserved for courtesans and actresses.)
I have some pieces with mixed diamonds and moissanite stones and I don’t think any amateur looking at the pieces could tell which was which.
The money you save on a lab grown diamonds will buy a lifetime of eyeglasses for you and for half of the short-sighted children in the world, and that might help you spot the difference…