First of all I look for girls that have a change in behavior. Like a girl that never moves off her heat spot is laying on the cool side all of a sudden. Or a girl that normally just slams rats has started refusing food. Now that doesn't mean that they all do that. I have some girls that will eat till with in a week or two of their ovulation. If they start refusing on a weekly basis for a few weeks I'd try pairing them up, especially if you've been cooling for the last few weeks.