If the girl in private life has any changes, it becomes always noticeable: it sunbathes eyes, the mood improves, she smiles more often. Many ways of definition of the fact that the girl fell in love are known or someone strongly is pleasant to her. To get to the truth it is absolutely easy, rather simple to take a detached view of her behavior.
If you hesitate or afraid to tell about the feelings directly, write the letter. Perhaps, it and will remain undispatched, but the fact that you after all could make a declaration of love, perhaps, will manage to bring together you with the addressee a little.