What enters my mind is entirely dependent on the person and the situation. It can't be a one size fits all scenario because everyone is different and every situation is different. Like if a guy walks up to me in a bar, it will have an entirely different meaning than if a guy were to walk up to me at a school.
I generally don't dismiss anyone unless they are extremely overbearing or obnoxious. If you come up to me all arrogant, there's a good chance I'm going to walk away. I don't have time for that honeysuckle. But, if you are polite and civilized, I will likely stand there and talk/listen. Little to no judgment on appearance. What's the point, for all I know, the dude is going to ask for directions. Actions will ALWAYS speak louder than words for me, so they will always have more power than words on me and determining what kind of person you are.
I don't generally assume. I can't possibly know what he's thinking, but if there are clues, maybe. I don't use the boyfriend excuse. However, going back to the obnoxious overbearing guy, I DO use the husband excuse. It's not a lie, I do have one.....he just lives with his girlfriend.
Trial period? I suppose everything in life has a trial period, but I don't really put it that way when dealing with relationship (platonic or otherwise). I would put it more like getting to know him better.