Be nice if there was a sure-fire way to tell the difference.
Wonder what would have happened if the home owner opened fire on the police with an AK47 and killed them all?
Easy. Look at the cop
If it's Harvey Keitel or Al Pacino, then = bad cop
If it's Ethan Hawke or Colin Farrel then = good cop.
If you are not in a movie, then you're on your own.