Oh that's a good question! It's so difficult to tell because everyone has a different way of expressing themselves, but here are a few generalizations from my experiences that may (or may not) apply to your situation.

1. I think body language and eye contact say a lot about a man's feelings -

   My partner calls them vibes, but he's from California

   - all of those unspoken cues that women are often good at detecting. For example, if he is affectionate toward you (giving you longer than usual hugs, touching your arm or shoulder, touching your face), that usually means something. Or if you find him making frequent and "intense" eye contact with you or gazing at you from across the room, that may be a clue!
2. Does he spend significantly more time with you than with other female friends, or give more attention you than to others when you are out in a group? That is usually an indication that he is genuinely interested in your company. (Being nice or friendly to people you come into contact with is one thing, but going out of your way to see someone or spend time with them is quite another... that takes deliberate effort!).                 Of course there are always exceptions to this rule... On the one hand, men who are very shy may actually pull away from the one they "fancy" because they are nervous or afraid to make a move. Flirtatious or extremely outgoing men, on the other hand, may appear to fancy everyone they meet because they give attention to just about everyone. So always look at the big picture to see how he treats other women. If he is the flirtatious type, then his attention may not have as much meaning.

   If its that complicated - why doesn't she just tell him how SHE feels.

   
3. Does he give you extravagant or out of the ordinary gifts? If you go out with a group of friends, does he offer to pay for your meal or movie ticket? (That has happened to me a few times, and I found out later that this was his way of turning it into an informal date!)
4. Some other things to consider: has he made it a point to introduce you to family and friends? How does he introduce you? Does he compliment you often (and sincerely)?

In general, if a man is going out of his way to spend time with you or to single you out from others, that is probably an indication that he is not just "being friendly" but putting forth effort to pursue a relationship or a deeper friendship with you. Hope that helps!

The problem with these rules is they assume guys know how they feel -- we don't