actually, if you evaluate the stifle conformation carefully you can pretty much pick out a horse, even a foal, that isn't likely to have a stifle problem.
Just watching the horse move isn't going to tell you a whole lot if you don't know what good stifle conformation looks like. You can have a horse with a weak stifle that doesn't lock up the day you look at him--perhaps the seller knows the horse has a problem and has carefully conditioned the horse to minimize the problem...so you aren't likely to see anything wrong with the movement when you look at the horse. But, you take him home and he stands around for a couple weeks and then starts locking up...or if it is a foal you buy him today and then a growth spurt two weeks from now has him locking up. Yes, growth spurts can cause a horse to lock up, but generally speaking those horses that have issues during growth spurts do have some flaw in their stifle conformation to begin with. A horse that has a good stifle almost certainly isn't going to lock up no matter how fast he grows.