I get the impression that it can be hard to tell mystics from low-grade mojaves ... There's a gentleman with an ad on the classifieds right now selling "possible mystics," because he bred mystic x mojave and couldn't confidently tell the babies apart!
I have a female mojave that looks, to me, EXACTLY like a phantom (I saw a big one in person at NERD, and honestly thought she could just have been my mojave gal) As Stratus says, it seems like it comes down to trusting your source with these morphs ...
The only thing I've consistently noticed in pictures of specters is the neck stripe. I know I couldn't pick one out of a box of normals, but then again I don't have one, so I really can't talk![]()