How do you tell the tonnage of an air conditioner if the BTU rating is not on the outside unit?

the model number will most likely have anumber that coinsides with feet such as12=1 ton,24=2 ton,30=2.5ton and so on. some upgrades may have numbers like 32 instead of 30, but that's usually on the inside unit the outside unit generally sticks with the 12=1ton rule