My favorite was having them log into every account right in front of you. Make them sit right across from you while you dig through everything. Remember that instant messages on facebook get saved under the message screen. Double check all sub folders in their email account(s) as they hide emails under the most mundane names. Check the phone records. If you have the money, hire a PI.
There is a radio station where I live that will say they won a prize of roses or chocolate to see who they want to send it too, all while live on the radio. Most all of them want to send the roses to the OW.
One DJ pretended to be the woman that the hubby was talking to on a cheaters web-site and called him for a hook-up and he fell for it. The DJ lady felt so "dirty" talking about it afterwards. The hubby was scum and everybody within listening range heard it.