There are tons of articles to read on saddle fitting - every horse, saddle, and discipline are different. I'd suggest doing a google search and finding reputable sources. Equisearch.com has some great info as well.
Basic things to check are that it does not tough the spine and does not pinch the shoulders but that would just indicate the saddles does NOT fit if one of those things happens. If both of those things are okay, that doesn't mean the saddle fits correctly though so you'd have to talk to a professional saddle fitter in my opinion.
I'm pretty bad at fit to be honest (so I used saddle fitter myself), but what even I can see is when there is a gap between the saddle and the back (you usually can tell it right away), when it doesn't lay nicely on shoulder (again - gaps or pushes against), and too wide (basically sitting on top of withers).
Just as a general rule of thumb. And IM NOT A SADDLE FITTER SO ALTHOUGH IT MIGHT LOOK RIGHT GET IT CHECKED.
Three fingers between pommel and wither when someone is mounted
Two fingers between gap under cantle and horses back when mounted
The flaps shouldnt impede the sholder and you should be able to fit ur fingers all the way around the flaps when you are mounted.
If its a decent saddle and ur saddler says it doesnt fit most saddlers will offer trade ins or part ex.