I am, unfortunately, not actually very good at noticing when potentially-bad things are happening, but when I do notice I try to check up on things. I need a better procedure for dealing with people I'm scared of, though.
Relevant personal history:
1. In college, while I was eating lunch in a food court one day, I saw a man and a woman (presumably a couple) having a loud argument in the middle of the food court. I went up and asked the woman if she needed any help. She said she didn't, so I went and sat back down and they left shortly thereafter.
2. Later in college, I lived in an apartment above some seriously skeevy people. They routinely entered and exited their apartment through a window instead of the door, and often played very loud music from their truck. There were always a lot of young men hanging around; my roommates and I didn't know which of the people lived in the apartment, but we were pretty scared of them all and never confronted them about the music or anything. At graduation, my mother and grandmother were walking back to my apartment with me, and passed one of the young men and his girlfriend having a very similar argument in the hall. They laid into him; I would have preferred to ignore the incident, because he was scary and knew where I lived. (In retrospect the thing to do here would have been to quietly go past them and then call the police, but I didn't get that option.)
3. Shortly after we moved into our last apartment, we heard a very loud argument between a man and a woman several apartments away. We called the manager, and he went over and talked to them.
4. At a party I hosted, somebody said something to somebody else that made the second person uncomfortable. This one I didn't notice happening, and didn't hear about until much later; both somebodies are actually reading this, so... somebody 2, if you would like me to have a talk with somebody 1 and make sure that doesn't happen again, say the word (and give me a few more details about what he said) and I will do that. I wish I'd known about it then so as to nip that sort of thing in the bud.
Generalities from said personal history:
Everybody, if somebody is doing things that make you uncomfortable at something I'm hosting, please tell me! That's information I need! I will put a stop to it! People who will not behave themselves do not belong in my house. (Given the set of people I know, severe cluelessness is more likely than malice, but in that case what we have is a Teachable Moment, not an excuse to continue the behavior. "He doesn't know any better" isn't even a good excuse for a dog.)
Even if I'm not hosting, I have no problem being rude if justified, so if you are being flustered by a lack of good response, you can come hide behind me and I will be rude for you. I can be good backup once I know things are going on, but I do not have particularly good social situational awareness with which to notice things going on.
The things I have noticed have been situations that were loud (therefore attention-getting), and also more potentially dangerous for an intervener than most vaguely-icky situations, because they involved long-term couples, and domestic disputes get nasty.
I am much less scared of confronting people who don't know where I live, even if they are really scary people (as long as there are other people around and the scary people don't appear to have weapons). I trust that if my attempt at intervention went to (threats of) physical assault, the other people would summon help. But if the scary people could easily find me later, I really don't want them to be angry at me.
Thing Two (linked from Thing One, but even if you're not interested in that, go read this): A Modest Proposal about sexual consent
I have nothing else to add to this one.