Legal or not, what you want is a radar detector - most devices either use radar, or literally laser-like beams, or they're CAMERAs in that they just record light that comes in - not send any out. Here's the problem with each:

Radar - requires special gadgets to detect the frequencies you're looking for. N900 hardware can't do it, as far as I know, and can't even come close.

Laser - is actually a tightly focused light beam. Either it hits the N900's light sensors, or it doesn't - if those sensors can actually detect the wavelengths used, can take it without giving an error from it being too bright, and can distinguish it from all the other wavelengths of light, you still have to get lucky and have the N900 get hit directly with the beam. And not just the N900, but the exact camera lens / brightness sensor / proximity sensor, which are the only three remotely likely to be able to detect it.

Camera - an actual plain camera, that just records light, is invisible in terms of emissions of the kind N900 hardware could pick up, unless it's in night vision mode and blasting infrared everywhere. In which case even if the N900 can detect those wavelengths, it can't necessarily distinguish them from a flashlight.