There isn't going to be a moment at which you will want to stop and ask for permission, "Hey, mind if we swap saliva?" So, you will need to test the waters to see if your partner wants to be kissed. When you have decided you want to French kiss, start kissing your partner with a slight open mouth, but keep your tongue inside. This lets your partner know that your mouth is open and that you are up for French kissing. Continue kissing with an open mouth and then, very slightly, touch your partner's lips with your tongue. If they respond negatively, stop. If they don't respond, then be patient. They might not be ready yet. If your partner mirrors your actions by opening their mouth, then they are ready to move on.