Buying a cheap laser power meter may cost more than buying a new 720mm laser tube. There are ways to measure the power without breaking the bank. It may not be super accurate, but you only need a thermometer. Warming up water is one way to test the power.
Remove the nozzle and lens; put a glass of water under the laser head. Measure the water temperature. Fire the laser at its rated mA. The laser head needs to keep moving; otherwise the laser will boil the same spot of water. Boiling should be avoid. Count for 1 minute, and check the water temperature.
Ignoring heat lost to air and the glass, we calculate the following by using the the water temperature increment:
Energy=4.184 J/g˚C*(Weight of water)*(Tfinal-Tstart)
Energy/60seconds=Power of laser tube.
If the mirrors are dirty you may lose power on them. Another way to compare power is using acrylic block about 2inches thick. Put it 5inches away from the laser tube output. Keep a fan/exhaust blowing so the block does not catch on fire. Then fire the laser for 6 seconds. Do this when the laser tube is new and good. A few years later we repeat the same process and compare the penetration depths. It is hard to get the exact power value, but we can know whether it is time to replace the tube. However, please note this is not a good way to compare different laser tube because of the laser beam profile difference.