Telling when somebody likes you can sometimes be quite tricky, more so if the person does not make their feelings clear in any way. In fact, attraction usually begins at the level of small, almost imperceptible signs.
In the first place, it is important to bear in mind that they may be extremely skilled communicators. There are others who, by nature, show a lot of interest and, through their attitude, they are capable of making the whole world feel worthy and important. That being said, the feelings that they have for you are not necessarily the same as those that they communicate to you.
When someone is really attracted to you, this can lead to a situation where they send you a lot of conflicting messages perhaps because being around you makes them feel a little bit nervous.