It is easy.
Women you find attractive, those you want to fuck and find sexy, are the women who are horny. Females who sends sexual signals(sometimes they dont even know it), are doing this, because they are telling males around them, that they are ready for mating and males catches these signals, by getting sexually interessted.
Does it makes sense? You cannot read it from the bodylanguage, but you can kinda feel it(our human nature). The girl who catches mens attention(no matter her look)in a certain way, is the horny one. If you got few friends in a club, you could ask them which girl they would like to fuck, if they say the same girl, she might be the horny one(but how horny girls are, varies a lot - it could be for a kiss, sex, touching, flirting, for fun etc. unlike men who just wants to fuck). Also, women in ovulation period, will look actively for a new partner.
"Ovulating strippers made an average of $30 more an hour than menstruating strippers and $15 more an hour than non-ovulating, non-menstruating strippers."
http://www.nickcampos.com/2010/10/ovula ... n-sort-of/