How do you tell if you're boring someone?

To elaborate, when I'm in a situation where I'm with someone I know not massively well (usually, a mixed group at a restaurant) you're in a situation where completely ignoring the people next to/opposite you would be rude (obviously). I.e., you're 'trapped' with a load of people. So you talk to them about whatever. The problem is, when I do this, after saying 10-15 minutes of conversation, I immediately worry that I'm boring them or even creeping them out. How do you tell if this is happening and hat would you even do if you were?