If your partner is staying super late at work, multiple times a week, all of a sudden, spending a lot of time with his work wife at the bar, or comes home smelling like another woman’s perfume...yeah, obviously that's sketchy.
Being a jealous person is no good for relationships. Ask for advice from people you care about and trust before dipping into homeboy's Facebook Messenger and emails. First things first, he has to have done something(s) worth being suspicious over.
Don't let emotions get the best of you
When the time comes to call this lying scum out, you want to be composed.
Instead of seeing something sketchy and immediately getting in your partner's face, take your time gathering evidence and laying out the facts. Then, prepare yourself. Meditate if you need to. Jot down some notes. You need to come to the argument with a hard, pragmatic attitude. You want to be Olivia Benson, not Jenna Maroney.