How to measure real laser power output?

Does anyone know how to measure real power output for the laser?
I want to buy 8 watt laser
https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=8+watt+laser&_sacat=0
but have a big concern if they are rated and have real power output.

there are many supplies in google: https://www.google.ru/search?q=8+watt+laser&oq=8+watt+laser&aqs=chrome..69i57j69i60l3j0l2.3482j0j7&sourceid=chrome&ie=UTF-8

so how to choose the right one?

any ideas?

submitted by /u/murnyanskaya
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