If you're asking when will they get their adult coloration (I can't think of anything else that yuou might mean by them "turning"), you'll have a pretty good idea when they get to and pass around the 100 gram mark, but they will still change on into adulthood (3-5 years+).
For me, I look at the parents (if possible), the side diffusion, and depending upon the exact morph, the coloration (as an example, for a lavender bloodred, I would want one that showed alot of peach).