Sealed with a kiss
Planning is always a good way to go - pfft to spontaneity - this is particularly true with dating, specifically when trying to work out when to go in for the first kiss. This first answer below will be useful for those without instinct or a gut and who want to kiss girls (NB this approach doesn't translate to males).
1. "To hit the right moment, make sure you're close to her and then observe her mouth and eyes carefully for one or both of the following signs: if she licks her lips or purses them, she is thinking about kissing you. If she looks down repeatedly at your mouth, she is thinking about kissing you. Once you have spotted this, let the conversation slow down and when there is a pause, crook your index finger slightly and use it to lift or guide her chin (assuming she is shorter than you or oriented slightly away), tilt your head, and then kiss her gently. Increase intensity [or tongue] depending on the degree of her response."
2. "Pause, smile, look him/her in the eyes. If he/she doesn't break eye contact kiss him/her. If she looks away and looks back, kiss him/her. If he/she stares at the floor, date's over."
3. Ask him/her.