Excuse me Sir, can you see the laser beam?

Hi,

I have a laser diode, looks red, probably about 660 nm, but I don't know 
the power. The laser spot is well colimated and easily visible in 
average lit room. Haven't had the chance to try outside in sunlight.

Now this colleague of mine is worried that after reflexions on several 
mirros (which are optimized for a different wavelength) the intensity 
becomes too weeak and the laser beam is not visible anymore when it hits 
the target. I estimate the reflectivity of these mirrors at the laser 
wavelength to be 40%. So after 2 mirrors, I have 16% left. I would argue 
the beam is easily visible, since the eye has such an incredible dynamic 
range (11 or so???). But he really wants to do the experiment to make 
sure the beam is still visible. Fine. He has a point. But my question is:

How do you defind the visibility treshold of a laser beam onto a target? 
When is the beam too weak to be considered visible by most observers?

rant on!