How can find out the capacity of a boiler?

Every boiler should have a stamped plate showing the amount of heating surface area, along with a serial number.

Many fire tube boilers are rated at five (5) square feet per boiler horsepower (verify with the mfr.) with a horsepower being 34.5#/HR from and at 212 deg. F feed water temperature. A boiler horsepower can also be determined by the boiler's output with 33,472 BTU representing a boiler horsepower. This is how hot water boilers are rated. The other thing you can do is check the BTU rating on the burner, and factor in a nominal efficiency (80%) which is what will be converted as output from the boiler.

The capacities of the safety valves are clearly set in limitations to the maximum allowable safe working pressure. Some boilers are fired well over their general design capacity, so it is important to know this. The capacity of a boiler of unknown manufacture or lack of other information, generally then falls to industry acceptance to estimated (not real) capacity based on square heating surface. The type of boiler, water tube or fire tube then dictates further the estimation. If nothing else is available in information for a particular boiler, then get out the ruler, measure and calculate. exposed surfaces all have value. Tubes, drum surfaces, any usable surface that transfers heat from the furnace flame to the water is contributing to the generation of steam.