Like most people, probably, I have several pet subjects that I love to talk about -- subjects that are sometimes interesting to other people, and sometimes not. Don't get me started on happiness, or the screening procedures in airports, or children's literature, or Winston Churchill, unless you really want to talk about it.
I made a list of indicators that I might be boring someone. Just because a person isn't actually walking away or changing the subject doesn't mean that that person is genuinely engaged in a conversation. One challenge is that the more socially adept a person is, the better that he or she is at hiding boredom.
Here are the factors I watch, when trying to figure out if I'm connecting with someone. These are utterly unscientific -– I'm sure someone has made a proper study of this, but these are just my observations (mostly from noting how I behave when I'm bored and trying to hide it):