I've wondered too how are we supposed to buy an aftermarket seat? We are spending a bunch of money on blind faith that it might feel right. How can we check out how a seat feels in advance?
If it's a seat you're not familiar with or can't check beforehand the short answer is you really don't know. Even within a manufacturing group they vary. I've had Corbin "dual tour" saddles on several different bikes. Three Road Kings, SEEG and SERG. Some they've been closer than the stock saddle, some higher, some lower, some about the same in some ways but not in others.
The stock saddles vary themselves from model to model. So the baseline itself is moving. Sometimes the aftermarket mftr can tell you specifically. But they often use seat pans to cover a wider range of models than a shorter term OE production that is made to in some way match a specific model.
Short answer, you don't know for sure until your butt sits in it.