Hey Fermina,
The easiest way to notice that is by examining how he interacts with you. For that you have to initiate less to allow him to show his true intentions.
See if he calls, asks you a lot about you and pays attention, inquiers about your dating life, doesn't talk about or doesn't go out with other girls, notices how you look, smell etc., remembers trivial things you did or said, tries to arrange situations where he can be near you or get you out of the company of other men.
These are just some, but the key thing is to differenciate genuene interest from other, friendy one...