CO2 and Yag laser power output is typically measured with a calorimeter-based measuring device. These types of devices measure the change in temperature of a black-body material over a specified period of time during which the material is lased. The change in temperature corresponds to an applied power.
The black-body material is usually a chunk of metal, such as black anodized aluminum. The black-body material is chosen to absorb as much of the laser energy as possible, otherwise there would be uncaptured power that could not be reported. For this reason, it is important to choose a power detector that is designed for the specific wavelength produced by your laser, typically 10.6 microns for CO2 lasers and 1.06 microns for YAG lasers. You will also want to be sure that the tool is designed for the power output you intend to measure. Otherwise, the meter may be off the scale or the resolution may be unsatisfactory.
These measurement instruments can be analog, resembling a cooking thermometer, or digital. The Mahoney Laser Power Probe is a common analog instrument that typically costs around $100-$200.
The digital instruments are more expensive but provide a more accurate readout. The costs for digital instruments start around $200 but can be in the thousands for advanced instruments.