[Alex]

zero,

Good question. In general professional porn videos have much higher production values than typical amateur home movies. That could mean better video quaility, fancy video editing, multiple camera angles and some even have a soundtrack. Also you can often tell if the people in the movie look like real amateurs (doing this for fun) or paid performers acting for the camera.

I know this isn't a perfect answer, but it's a good starting point.

Thanks