In a fascinating study by Kristina M. Durante of the University of Texas, Austin's Department of Psychology et al., entitled "Changes in Women's Choice of Dress Across the Ovulatory Cycle: Naturalistic and Laboratory Task-Based Evidence," researchers identified a finding that's at once something you probably already guessed was the case, but never saw hard evidence for before: that when women want sex, they dress for it.
Here're the findings of the research:
“Although each data source supported the prediction, the authors found the most dramatic changes in clothing choice in the illustrations [of what each woman would wear out that night]. Ovulatory shifts in clothing choice were moderated by sociosexuality, attractiveness, relationship status, and relationship satisfaction. Sexually unrestricted women, for example, showed greater shifts in preference for revealing clothing worn to the laboratory near ovulation. The authors suggest that clothing preference shifts could reflect an increase in female—female competition near ovulation.”
In other words, when women want sex as they approach the time of the month when they're fertile, they dress more provocatively to attract more male attention and out-compete other females.
When it comes to ways to tell if a girl is horny though, this is just the tip of the iceberg.