I'm going to guess that if you see someone pitching a tent on the sidewalk or sleeping in the doorway of a business, there is a fairly good chance they are homeless. Sleeping in a car doesn't necessarily mean anything, but if it is full of otherwise household items, the chances of the person being homeless goes up.

You can also, you know, ask. Sure, a few folks might lie, but it is better to let them get away with the lie than to punish a bunch of folks with no choice.