It's been my experience that most things that seem like signs a person might be married or in a relationship, tend to indeed be signs. In fact I just assume that everyone has some relationship or another going to some degree until they prove otherwise through a solid combination of specific non-suspicious statements and behaviors.

I've come to prefer asking the question "how single are you" instead of just "are you single", because I've found that it's easier for people to lie in the form of a one-word answer, especially if in their mind they might be thinking "not really" because they are thinking they are not really into the relationship they're in, or that they are "about to" break up, or some such, and the shortened version of that to them can be "no". The first question advises that I am smart enough to seek out nuanced and accurate information, while still being a simple enough question. It's one I like to make sure I ask in person, or at the very least on the phone, so that I can witness their real reaction; emails don't as much lend themselves to honesty, where people can cloak their emotions and take their time concocting clever answers.