How do you find out if you have a powder chamber? First, look at the manufacturer's literature, or their website. If the gun is made by Lyman, Cabelas, T/C, Investment Arms, Pedersoli, or any of the European companies that make the guns for American companies, it almost certainly has a powder chamber.
How Big( diameter) the powder chamber is for a particular caliber, varies from company to company. The .50 and .54 caliber guns usually- but not always-- have a powder chamber cut with a 3/8" ( .375 caliber) drill. When you go smaller, the powder chambers tend to also be smaller. Those on the small caliber guns( ie..32& .36) often range from .25-.27 caliber.
You usually have to run a variety of rods and jags of different diameters to find out what you have. Using a bore brush of the right caliber, putting the cleaning patch in front of the brush, so that the brush grabs the patch with its bristles, and drives the patch down into the powder chamber, to clean it, makes much more sense than trying to get a jag of the Exact size, IMHO. Some of the .40 and .45 caliber barrels fit a .30 caliber size brush to clean their chambers, for instance. Use pipe cleaners to clean the flash channel under the nipple( remove the nipple to gain access, and/or remove the "clean out screw" if you have one for that purpose).