it would be difficult to tell especially with emotionally cheating but definitely can tell if he is putting less effort into the family/relationship

The change in his behaviour and attitude towards me are the tell tale signs

my partner sucks @ lying, so its easy to know when he's lying.

The subtle changes and some other symptoms.

Maybe.. monitor the little changes in him

Definitely yes as the behaviour changes

Trust is important in any relationship

you will notice his changes though

I think you’d have a feeling

It would be obvious i feel.