There is a fine line between "Secret Admirer" and "Stalker"... I would not recommend being either. If you like someone you should just go and tell them.
He/she looks at you more often or if he does then look at him back.
He/she seems curious to be with you more often.
The person usually talks to you and makes you laugh.
That person is making you...
It is suppose to be a secret, but usually the person that has a crush on you will give it away by taking quick glances at you when they don't think you're looking. If you catch them looking then...