The general rule of thumb I've always read/heard is does the trigger hand placement naturally allow the finger to reach the trigger if you were to set the butt on the inside of the elbow - stick your arm out to your side, bend it so your forearm is pointing up, set shotgun/rifle on your arm out by your elbow. Does your hand naturally reach the grip so you have proper trigger position? If not, you either need spacers or to have the stock length shortened.
The second thing would be your non-trigger hand reaching the forearm. I have short arms so I just have to deal with most pump/SA shotguns and hold it as far to the rear as I can. You normally want to have the supporting arm to have a fair bend or you'll tire quickly if shooting competitions. Not as big a deal for hunting where you're not shooting for much time.
Third thing is sight line. Is your eye aligned with the sights when you naturally rest your head on the comb? Do you have to tilt your head up, down or to the side? You may need to add or remove material if it's not adjustable (most aren't). This is more important in competition where you must be fast.
Fourth thing is balance. Most shotguns are barrel heavy, which helps facilitate smoother swings while tracking birds, but some could be uncomfortably heavy for weaker or smaller shooters, especially if they'll be shooting for longer sessions. If you can stand in position for a couple minutes with no discomfort it should be OK.