"Seeing a laser beam" is a misconception. You can't see it directly, only its effect.
Whether you can see the path lit up by a 10mw green laser depends on how efficient the laser is, how dark the night is and the density of the reflective particles (moisture, dust) that reflect the light that shows the path.
A practicle answer to your question however is that on most dark nights that laser will show a beam that you can see.
If you are near a fairly large city, I live about 12km out of Melbourne, you probably won't see a 5mW laser, a 10mW should be visible to people standing nearby. Further from a large city a 10 should be visible easily by others. On a Full Moon night, you would be hard pressed to see anything much below about a 20mW. I have a 30mW mounted to my scope for showing people where the tube is pointed and this is visible even during a Full Moon on a school footy ground.
I have a 5mw laser and you could see it from miles away, and i live 15 mins from sydney cbd. If I can see it reflecting of airplanes when they fly past then you should have no problem with a 10mW laser .
Ps, only kidding about the airoplane thing.
PPS really, only kiddng.
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