What about trusting your almost ex? I think the biggest issue is actually trusting your soon to be ex. I know that seems harsh but coming from someone that has been down that road, if I had to do it ...
If your ex was a trust worthy person when you were together then i see no harm in trusting your ex know. If your ex was always liying when you were together then maybe you should not trust them. IF your ex has a history with you of being honest and up front with everything the go ahead and trust your ex.