I'm with ONR. There are the obvious ones that you can decide eaaily. Head knock, bleeding etc, get someone on quick. Or a little knock on the foot in a careless challenge, the player wants to get on with it, a quick "are you ok fella?", yeap. Get on with it.
It's the ones in the middle you need to be careful with, slow it down, look for visual clues, look at the player, look at the bench or team mates, talk to player, ask the manager if you think is a good idea. You decide based one what you think they want you to decide. Obviously also keep gamesmanship in mind.