You can't. It does not cost you a fortune, but compared to your new posession everything is expensive.
These instruments are not rocket science (other than the Laser itself). It boils down to absorb the entire light (i.e the surface has to be mighty black) and to transform this energy into heat or electricity (for really low power and defined wavelength) that can be measured. There are DIY recipes here how to build a Laser Power Meter (LPM). But in the end, you will still have to calibrate them against a known sample or you can throw dices again.
I throw my dice and it says:
Your Laser is between 50 and 70mW.
Wild guess, but many of the "cheap Chinese <5mW class IIIb burning torches" Lasers fall within this range. It is unlikely to be lower than 30mW and similarly unlikely it will exceed 100mW.
You can do a quick test. Paint a match black and point at it with the Laser. If it lights quickly, you are certainly in the >1W range (highly unlikely). If you can focus the beam and even then the match won't light, no matter how long you try, then you are certainly below 10mW.
Would you need more precise numbers? Would that change anything? Keep the ray (even reflected) off your eyes and the eyes of every other living creature and everything should be fine.
If you get the impression that it is "damned bright" and you can't easily look at the spot on a light surface - get a pair of proper Laser Safety Goggles soon. Chances are, that the beam IS dangerous to the eye and you have just two of them.