Aside from going up to them and going "Is you horny darling?", I don't think it'll work. Maybe if you said in an Ali G kinda voice...
I don't know anything about estrus, but surely there has been some kind of study performed concerning the subject? things like hormone levels, temperature, days since last period etc. will prolly all play a role.
Why would you wanna know when they're most sexually aroused anyways? It's so much more fun getting them redhot from a cold start
