The way I've been told is to hold the saddle pommel down against your hip/thigh and put a little bit of pressure on it (as if you were trying to fold it) and it shouldn't move.
Sorry I'm not great at explaining it
ETA: Just found this- basically what I was trying to explain
If you're just looking at photos, if there are folds of leather on the seat, or the leather is loose/crinkly that is also a good indicator of a damaged or broken tree