Try going to a "Real Gun Shop" and it sounds like you should take your handgun in with you. The caliber is "usually marked" somewhere on the barrel.

But, I'll also suggest you fine a good place to take a firearms class and learn a whole lot more about your particular firearm. If you can't find one try contacting the NRA 'on-line' and they should be able to put you in touch with a firearms instructor in your neighborhood.