Normally, it's on the barrel.
By the serial number. Try proofhouse.com
"My pistol is .40 caliber." "We were fortunate to find a manager of his caliber."
Well, my "piece" is a 1995 Smith & Wesson "360" , which if I'm not wrong, is a 0.360 caliber "six-shooter" western style revolver. There's no cartridge which might hold adiditional bullets. Six Baby and no more! Upon further review, the guy wanted to know how to manufacture date, something I can NOT answer, at least not right now. I also found out since my previous answer that my revolver is actually a S&W 359 magum, (0.359 caliber plus) but I swera the sales guy called a Model 360 !
where can i find replacement parts for a 22 caliber Stevens modle987
45 caliber Machine Gun and 50 caliber machine gun
Caliber and bore are used interchangeably nowadays. There are technical differences you can find in the dictionary if you wish.
It will usually be marked somewhere on the barrel.
I have a 32 caliber hammerless Iver Johnson serial 45158 with a 3 inch barrel how do i find out about it
Find a gunsmith and ask to be shown how.
Put it on a scale
In the owner's manual
Try on line auctions.