I think with affairs it might be clearer that they are going off to meet someone, or guarding their phone, changing how they dress or picking holes in you and your relationship so they can discard you when the time comes but if a man just "checks out" of the relationship he can be present and not having affairs but just not actually involved in the relationship anymore or family life if kids are involved.
What signs would you say were present if a man has checked out of the marriage?