How much evidence do I need to prove my spouse is having an affair?

It is not necessary to catch the third party and the cheating spouse in the act, but there must be evidence that the unfaithful spouse was inclined toward a sexual relationship with that third party and that the spouse had the opportunity to engage in a sexual relationship with that third party.  Email, social media posts, suspicious behavior and statements from coworkers or friends can all be used as evidence of infidelity.