[I DIDN'T KILL MY WIFE]

Invite them out for drink, they buy her a couple of drinks which will cause her to go to the toilet. And once she finally does that, you take a

LONG HARD SNIFF

of the chair she was sitting in. Trust me, you'll start to recognize the smell of ovulation after a while. If you notice she's not ovulating you can just next her right then and there to be honest