how to find a laser beam Let's say that you want to know if there is a laser beam around a big area, like a stadium. Or an airport. How could you detect and find a such laser beam (could be by a laser pointer or something more powerfull) ? If I take a laser pointer, I don't see the beam but its reflection. But if you are in an open space, how could you do that? 
 A: If there is no "beam spot" on a wall, or reflection from a surface, you are left with scatter from water droplets ("fog") and dust particles, as well as molecules of the atmosphere.
The small objects result in Rayleigh scattering, which is the reason why the sky is blue; while larger objects result in Mie scattering.
The polarization of the scattered light can be used, in theory, to help locate the light source; some of the formulas are given here: http://physicsx.pr.erau.edu/Courses/CoursesF2008/PS495C/Rayleigh.ppt
In order to see this scattered light requires sufficient contrast, which is very unlikely during daytime, but is much greater at night.  For example, I can see the path of the laser beams I work with when all of the lights are out, but not when the lights are on.  Thus I am mostly in the dark about my work.
Tracking the light source has been done if the polarization can be sensed; bees do this every day.  Some examples are described here: https://www.polarization.com/sky/sky.html
The result is that unaided vision is unlikely to spot a laser beam in daylight, but if remote sensing techniques are applied, it is possible.
A: Since these laser devices are typically very narrow linewidth (ie. high spectral brightness), you can use a simple photodiode with a tunable filter to filter out other light sources, and have fairly high probability of detection of the laser light from any direction because of scattering.
