Unless you know the line of fish, you cannot tell what potential they may have. You can influence the body bulk and wen development to a degree with feeding and such, but genetics will dictate to what degree.
A nice, consistant line of fish that produces a consistent percentage of nice babies is a great (and unusual - at least here) thing to have. My lines are so diverse - having to use unrelated parents to start them, that I have to go through hundreds of fry to even find any that are slightly close. To tell you the truth - you are simply trusting to luck what you will get from a fish-store. Most of the fish there are bred to fill numbers - not necessarily breed specifics. You may luck out and find the culls of another breeder - and find a good selection if the breeder is pretty far along in creating a solid line of fish.
I am glad Riz posted in. He REALLY knows his stuff with the TVR, SVR and Lionhead!

Hey - I should take a picture of that fry - the one that was left in that other post. He is growing well - but still not developing the type of wen I would like to see.