Too get real close get a ohms meter set it on ac volts. Put the test leads on each out put from the amp. Get a reading. If you get 25 ac volts then 25x25=625 now divide 625 by the ohms of the sub. 625 divided by 4ohms = 156 watts. Without any test stuff you can add the amps fuses total and x 13.5 this will give you the max watts of the amp. Take the max number and divide it by how efficient the amp is. Class D 75% other amps 50% is close.
This should not be done to see how many watts your amp is putting out at real loud volume. It can damage you ears and your subs. The head unit should not be turned up as to clip the sub. Also a subs ohms will change when playing songs or test tones and give a wider range reading. Dummy loads in the ohms you need should be hooked up to the amp instead of sub to get true reading. I do it with songs and get the highest reading. This way it's just harder to read because the needle moves up and down. Test tones the needle will not move and is easier to read.