Follow your insticts ye, but sometimes we women tend to be in denial, that kinda blinds you. Here is a few pointers:
1) Dress sense changes, starts to groom himself, new hair cut, perfume, cologne e.t.c
2) Excessive showing of affection or reduction as the case maybe
3) Fault finding, short temper, criticism
4) Working late, office dos( you never get asked to come along), meeting office mate for drinks
5) Secretive, phone calls end when you enter the room e.t.c
6) Usually leave the house after a phone call
7) Phone and laptop passworded.

Not allowed in his car without him being there( cheating guys leave all there cheating paraphinelia in their cars)
Hope this helps a bit,