Their Phone Can Give Evidence
Their cell phone will be the best place for “proof” and so it’s important that you search it for any numbers that your partner keeps calling / messaging. Gather as many “unidentified numbers” as possible from the phone of the cheating spouse.
Just look through their incoming / outgoing calls, text messages and address book, noting down all the numbers you don’t know or look suspicious. And find out who owns each number. Using the list you made of the numbers you don’t know, perform a reverse phone lookup on each number. This will bring back information including full name, address, line details, carrier information and further background information.