You either make your intentions clear from the start, or you allow yourself to fall into that situation.
I think it’s pretty sneaky/conniving to pretend to be someone’s friend while you have feelings for them in hopes that they’ll eventually give you an opening. Especially while they’re in a relationship.
As soon as one of you are romantically attracted, the friendship is already over. Whether you’ve admitted your feelings yet or not.
Always be honest with your “friend”.