It's easy. Get a big sheet of clean paper, stand a couple of meters away, and shoot using your sights. Look at the pattern the shot made on the paper, particularly on how wide the pattern is from edge to edge, how evenly the shot is distributed (all in a clump with some outliers, or even grid-like dispersal), and how close to your point-of-aim the pattern is centered. Then repeat at several distances, say 5-10-15-20 meters. For man-stopping work, you want 80% of your shot to hit a man shaped target with most in the center of mass. This "pattern" with vary widely depending on what type of shot you use and from what manufacturer you get it from. The idea is that you find out what shot gives you the smallest, most consistent pattern at various ranges and you stick with that type of shot. Kinda like sighting in a scope for specific ammo. For slugs, you can go out to 100-200 meters and see how accurate you are, some slugs are very accurate in YOUR gun, others won't be, but you never know until you do it. Ghost ring sights can eliminate some of the variability since I feel it is more consistent from shot to shot and user to user, while the bead front sight can vary dramatically a range depending on how you present the gun, clothing, etc, and is very different from person to person. Needless to say, this can be time-consuming and expensive if you try a lot of different loads, but it is the best way to get to know how effective your gun is. Most casual shotgunners are very suprised at how close they really need to be to get a good pattern on a man with buckshot. For upping the "wow" factor, let your friends play around, then bust out a shotgun from Vang Comp that can pattern OO buckshot on a dinnerplate from 15 meters. Very nice...... Hope this helps. Later.
-edited for moronic spelling mistakes