Paying attention to the girl's eyes can help you find the sweet spot for speed. When you're moving at exactly the right pace, the girl will open her eyes to look at you. This may depend on personality traits, but has been confirmed to work for most if not all characters. If she closes them again, she's likely moved past that speed on her excitement and carefully working the mouse wheel forward should find the new, ideal pace again. Some girls tend to like things slow, so if speeding up significantly has no effect, try dropping very slow again. If she's interested her eyes will be at least half open, if she's REALLY into it (not always an option) her eyes will be wide open in shock. If she's really mad with lust (seems to be based on keeping her eyes open for most of the time) then she sticks her tongue out and starts panting. You might want to make the guy invisible using (1) if you have a hard time seeing her eyes.