Check at your local newspaper - you can buy (real cheap), or get for free, the roll ends of the newsprint. make a frame that will hold up a 3-foot piece of it, and shoot it at a known yardage. Some gun clubs (the one I belong to, for instance) have patterning setups. Our club has several big steel plates on pipes in the ground - you give 'em a quick shot with a spray can of white paint, let them dry for a half-minute or so, and you're ready to go.
What you are trying for is a load that spreads the shot uniformly in a circle. Some loads will shoot a ring of shot that has almost no pellets in the center, others will do just the opposite, and some will spread the shot wider than it should for the choke used.
Often, a very slight change in powder charge or wad type will make a dramatic difference in the uniformity of the pattern. Also, a load that patterns well in one gun may not (actually, probably won't) work well in a different gun.