You don't necessarily have to ask her, but you need to make sure she wants you to kiss her. You do that by reading her body language and seeing how she reacts to your advances. For example, you're sitting down with a girl in your arms and you want to kiss her. You make a slight advance, like rubbing her inner thigh, if she seems to enjoy it, go ahead and make another advance. The next logical advance would be to gently carress her neck and earlobes with your fingers. If she exposes more neck and obviously enjoys it, look at her in the eyes, move her hair from her face slowly and move in slowly for the kiss.