What signals does a girl give if she only wants to be friends?

Communication is 90% of any relationship. Talk to them, trying to use signals is not a good idea as signals often times get misinterpreted. It's better to come out and say it, that way they know where they stand. If he likes you, it's a good time to setup that boundary, if you don't, like turns to infatuation and so on which makes it difficult to go back.