Pshhh... you're thinking about this in the wrong way.
It's not like, "girl puts head on shoulder --> execute Z maneuver."
Her head's on your shoulder, she's kino'ing you. Is this in public? In private? How long? What's the mood? In any case, if a girl is touching you, don't think, just escalate. Grab her, turn her towards you and hug her. Tease her a little, mess up her hair. Get playful. Get sexual. Whisper in her ear. Wrap your arm around her, grab her tight, rub your hand firmly up and down her side.
And whatever you do, don't let her put her head on your shoulder in a friendzone kindof comfort space way. If that's the case, tease her, mess around with her a little bit. You don't want a girl to be that comfortable with you that she could fall asleep on your shoulder - you want her to be startled with the knowledge that at any time, you two could wind up f*cking each other. After you're hooking up, sure - the head&shoulder thing isn't bad.