If I'm looking for people to follow I tend to pay attention to when they last posted a hub - as well as their frequency of posting hubs in the past (for example, did this person write a bunch in a short time period and then disappear, possibly for good? Or do they generally only publish every few months, meaning I'm likely to see them again? Not a foolproof method, obviously, but it's a place to start).
Just to get an idea of how much they frequent the site: on a Hubber's profile, right under their name it should say "Last activity *insert time here*"
Then (also on profile, though I gather from what you said that you know where everything is) I like to click on the actual "My Activity" button. It shows forum posts, questions, answers, comments, etc. It's a great indication of how involved in the community aspect the person is or isn't.