Is there a way to measure the power a laser’s tube puts out?

So how much power is your tube really putting out? If you need to know the answer, the only way to measure is by using a laser power meter. Two reading points are usually made to determine the power available on a given laser system. The first and most accurate is that measured as the laser exits the tube. The second and easiest to measure is the power on the table after the beam has reflected off the mirrors and through the focus lens. Although the latter may not be the closest value to the rated output of the laser, it can be telling of failures of the beam alignment of the mirrors or another problem in the optic path.

—Coherent