You should have a strong reason to believe that a guy is into you when he actually pays attention when you are talking. Under normal circumstances, guys are not exactly renowned for their listening skills. However, when a person they are interested in is talking, they find themselves going in a trance. If you notice a guy giving his undivided attention to what you are saying, then there is a very strong chance that he is into you.
If the person you are interested in calls you out of blue and does not really have a concrete excuse for having a conversation with you over the phone, then there is a strong likelihood that he may be interested in you. You would probably hear him asking you stuff such as “How are you feeling?” or “How was your day?”
Guys do not ask these sorts of things from girls they have no interest in.
If a guy who is normally pretty confident starts to get a bit nervous or feel shy while you are around, then there is little to no reason for you to believe that your presence does not have an effect on him. His sudden shyness or nervousness results from his desire to do something to impress you, or at least avoid doing something that might make you find him weird.
If you notice a guy going out of his way to try and lighten your mood by making you laugh when you are a bit down, then understand that he may have feelings for you. You might find him cracking jokes in your presence to catch your attention or seek your approval.
Do you notice the guy staring at you during class or in the hallway? Does he regularly stares at you? Do you see him staring at other girls the same way he stares at you? If the answer to the first two questions is yes and third question is no, then the guy is surely having trouble taking his eyes off you and is definitely into you.