Without directly asking someone, it can be hard and sometimes impossible to know if he is interested in you. It's not always easy to read people as we are pretty complex, but there are some signs that point towards someone being interested in you.
One of the main tools to know if someone is interested is by working out their body language. Someone who makes themselves approachable, is very tactile with you and makes his body posture look tough is usually trying to get your attention. In contrast, someone who doesn't want attention will usually be quite tense.
A form of non-verbal communication is eye contact, and it is a very powerful tool when it comes to flirting and can indicate attraction. If someone avoids your eye contact, it's not a great sign, but if all their attention is on you and their eyes keep catching yours, it's a subtle and subconscious message that they're interested.
Someone who talks a lot and rambles when they see you is usually because they are trying to make a connection with you, but aren't sure how to go about doing so. It can be particularly unnerving or intimidating going to speak to someone they like and making conversation. However, there efforts demonstrate they are trying to make a bond and impression.
Some people aren't sure how to act when they are around those who they 'like' and can come across as having a nervous manner when in fact, they are just anxious about making an impact on someone.
If you find you've been bumping into the same person, it's because they are trying to find any reason under the sun to be around you so they get the chance to flirt. They'll make an effort to be at the same places and to find out where you're headed to. It may sound a little 'stalker-ish' but they are merely trying to get you to notice them.
Is there someone who always sticks up for you and is always taking extra care to look out for your wellbeing? They notice things that others won't and take extra interest in your likes and dislikes. They may even change their appearance to impress and so that you'll notice.
Some hints are subtle, while some can be full out flirting. Pay close attention to people's behaviour and signals that they give off and go on your instincts.