I mean conventional wisdom would be someone who is sitting with an open posture and looking around at other passengers.
If someone has headphones in, and head buried in a phone/book, that's a good sign that they are closed off to conversation and want to be left alone. I appreciate that this is not true for everyone. But if someone were to engage with me when I am closed off like this, they are unlikely to receive a friendly response.
Side note, I don't believe this is true in every situation. For example I don't believe the above applies equally to someone who is sitting next to you on a long haul flight. Chances are they are just very bored and entertaining themselves in this instance. But if it's on public transport or at a bus stop, then yeah, they probably aren't interested in anything you have to say.