There's been a lot of talk about fit, feel, drop and cast that may, or may not, confuse our seeker of their first gun. I may well be teaching granny to suck eggs but here it is anyway.
The fundamental difference between a standard shotgun (be it side by side, over and under or single barrel of any typ) and a rifle is this. A rifle has a backsight and a frontsight. A shotgun has no backsight and for a frontsight it has a small round bead that may be metal or coloured plastic.
So in a shotgun it is your eye that performs the role of the backsight. Thus the purpose of correct fit, feel, drop and cast is to have the stock of the shotgun of such dimensions that when you bring it to the shoulder your eye is positioned where it will be centred and aligned with the barrels.
That your eye is looking straight down the barrels parallel to the bore left and right and perpendicular to it up and down. So that distant where you eye looks directly at is the same distant point the shot will arrive when you fire the gun.
Too much cast (the parallel) and you eye will be looking to a point right or too little looking to a point right of where your shot will arrive when you fire the gun. Drop is like the two hands of a clock. Too much and the gun will shoot low of where your eye is looking. Too little and it will shoot high of where you are looking.
So how do you check? There's the old advice of looking at a fixed point and closing your eye as you bring the gun to your shoulder and then opening it when the gun comes to the shoulder. Or the other old advice of doing it to a mirror and seeing where your eye is in the reflection in the mirror.
To my mind BOTH the above work for a gun that already in fit, feel, drop and cast is correct for you. But that in one that isn't you'll subconsciously correct yourself and then apply a physical adjustment to your "aim" that you then take as being a sign that all is correct. But you are fooling yourself.
So thus the only real test is indeed to try before you buy and an easy to hit straight going away and a straight incoming target. Then an easy left to right and right to left crossing bird.
So yes handle the gun dry (the fixed object or the mirror) to discount any that have clearly totally the wrong fit, feel, drop and cast. Put them back in the rack and leave them there. And of those that however do seem to be right (for you)? Do then try them on actual birds be that clay or feather.