If she's not expecting it, but it feels right to you, then I would say just go for it and kiss her and see what her reaction is. But if you're both, say, sitting close to each other, having a personal conversation and you're both looking in each other's eyes, then she'll probably lean in to kiss you if you just lean towards her too.

And after that has been initiated, then you really just need to go with the flow, try to feel what she's doing and what she wants, and also what you want.