Set a target up at 3-5 yards, and dump the mag or cylinder. Do this with various calibers. Observe your patterns, and consider how confident you would be in putting shots on target with speed.
This can be help informing which caliber is right for you.
Currently Own: Beretta PX4 9mm, Glock 23 (Gen 4), Glock 19 (Gen 4) x2