Go outside or to a window where you can see the Moon clearly in the sky. Close your eyes and visualize the person you desire. Chant:

"If you love me, let me know.
By this charm, his/her love to show."

Next time you see the person, notice any form of new behavior they exhibit. If the person seems to talk or look at you more often than usual, see this as a sign that he/she feels love for you.

A Word of Caution: You must be sure they have feelings for you in the first place for the spell to succeed.