After 3 of these, I should have paid more attention to their reactions to being asked an innocuous question out of curiosity, with no jealousy or suspicion intended. Floundering around, turning a funny colour, denials, ludicrously detailed or unbelievable explanations for something that should have been quite simple. Little things. Not believable at the time, but not quite enough to believe in an affair. And oh my, the angry indignation if I pointed out their answer seemed unlikely.
Eg, got in car passenger seat, it was in a different position. Simply commented on it as I moved it back and asked who they'd given a lift to. Just conversation really, expecting them to talk about a friend. Denial it had been changed, then a convoluted unlikely story.
Came home from playing cricket at 1am pissed as a fart. Weather had been poor, took one look at him and knew he hadn't been playing, thought he and the team must have gone out on a booze up instead. Not great, but not a problem. He came up with a stupid story of the cricket being called off and deciding to go for a walk round the footpaths and fields instead till midnight, and he wasn't drunk, he'd just had a couple with the lads first.