You pose a very good question about a very real problem. As you have realized, half the battle is knowing where to watch and what to watch for. It's something we all have to learn. I spent a few years training volunteer officials for a church league. Teaching them how to deal with clueless, stupid, out of control rough play (i.e., church league basketball) was was a major challenge. I assembled what I think is a useful and detailed document on this topic. You may find it helpful. You can find it here:
Controlling Rough Play.
Hopefully it will give you some of the guidance you're looking for. If you have questions about any part of it, or if there are topics you'd like expanded on, bring your questions back here and I'm sure you'll get feedback o'plenty.