There are definite signs that you are on their mind and they are thinking about you more than just when they see you. The most important thing to remember is that someone has to be thinking about you before they take action towards you. This means that in order for them to talk to you or approach you out of nowhere, they have had to been thinking about your for a while. You haven't just popped up in their head.
Here are a few definite signs that someone is thinking about you.
They phone you unexpectedly - If he or she calls you out of the blue, for no reason, then that means you were on their mind; especially if you don't talk that way very often to them. They were thinking about you, and they probably took a long time to pick up the phone to call you. This of course includes random texts, pokes, tweets, or any other form of communication.
They approach you about things you are interested in - If they come up to you and start talking about your favorite band, even though you have never told them who your favorite band is, then chances are they found out from someone else and they have been thinking about using it for conversation, or to get your interest. If this happens more than once, then it's a guarantee they are putting a lot of thought into you and your interests.
They ask about you or mention you to others - If your friend is telling you that someone is starting to ask about you, or is starting to talk about you, then that someone is also thinking about you. This is obvious right? They are trying to figure you out and they need some questions answered. Who better to do it with than people who know you?
On the other hand, if they can't seem to stop talking about you and your ways then they are putting a lot of thought into you and your ways.
They are always around you - Whether you work with them, go to school with them, or are in the same circle of friends with them, they are constantly close to you. If you are in someone's line of thought then they will also have you in their line of sight quite often. It's much easier to think about someone while you are looking at them. And if you have been putting a lot of thought into someone, then you want to be around them.
Think about the people who you don't think about often. Are they around you often? Or are you not really sure where they are? Chances are you don't know where they are. But you will know where the people you think about are, and you will gravitate towards them.