Lasers are nothing more than light, so you essentially have to calculate the number of photons that are falling upon an particular pixel, which you can then translate to a number of electrons in the pixel. "Damage" only occurs when the number of electrons exceeds the well-depth of the pixel (measured in electrons).
But the damage that occurs really depends on how much over the well depth you go. If you generate 55,000 electrons in a single pixel that has a well-depth of 50,000; you're pretty alright, but if it was more like 400,000 you would start to have a problem.
Also, it depends on how long this occurs for and how often. If you do it once for a millisecond, you're fine, if you do it for an hour, you probably messed up your sensor fairly well, or at least that column in the sensor.
Shining a laser on a sensor that is not turned on will not do any damage, assuming you don't generate enough heat to wreck the sensor.