When you shoulder the gun, if it has a rib, you should not see the rib. If it has a mid-bead and one on the end of the barrel, they should align like a figure 8 when you have the gun properly mounted. once you practice a few mounts, try it again with your eyes closed after you determined a predetermined spot. (In the store, maybe an overhead camera or something)..If you can, with your eyes closed, mount that gun and then open your eyes and see the perfect alignment of no rib, etc, it would be fairly close. The absolute BEST method is a custom fitting session, but that would not be economically feasible at this point in time. Some folks like to mention putting the gun into the crook of your elbow - that is very crude at best.
My personal advice would be to go to a local gun club, see if they will rent, or you can borrow, several different guns - members LOVE showing off THEIR guns.....find one that fits, or see if someone there can help. You can also check out their clubhouse bulletin board - as many have guns for sale by a member. You'll tend to get a fair price and an honest indication of what is the real condition of the gun - after all, he has to shoot there, and he screwed you royally, his rep would suffer.
Take your time and find what you really works - and in the meantime, save up some more pennies just in case THE gun comes along and it's a little out of your range..
Good luck