With bullet cameras, this is easy. You look which way they are pointing and then either guess their FOV, or even look them up based on how the model they appear to be.
Most dome cameras are behind tinted glass and it's hard to see where they are pointed at.
Knowing where a camera is looking is important during reconnaissance because it lets you stay out of sight later.
Bonus question: Is there a way to track this in real time for cameras that can pivot 360 degrees within the dome?