It's a matter of experience, reason, guesswork, research, and luck. The most important part is to not begin by assuming that the video/photo
is actually of a UFO then take it from there. "What could it be?" "Where is the video taken?" "What were the circumstances?" ...
Comets do not visibly move across the sky (unless they are very close to us, too close). They change position from night to night like the planets do,
but faster.