That's the challenge of sending and receiving unsigned love notes. The receiver has to guess who the sender is; however, their guess may be very wrong! There's also no way to respond if you don't know who sent it.
The best course of action is to either wait until the next letter or jumpstart the process by trying to determine who sent the note so that the person will reveal himself or herself more quickly. Talk to friends, sharing that you received a note and you are trying to discover who sent it. See if they know who might have a crush on you. Out of privacy and respect for the sender, don't let others read it.
Sooner or later, the sender will step forward. You're going to have to be patient on this one. Have fun with it!