Your partner smells like perfume

Perfume, hair other than his own, the handkerchief with lipstick, just check his clothes, wallet, palm pilot, cellphone records, credit card records for motel and restaurant bills, internet history, his car for suspicious items like foldedback seats, blankets, condoms, hobbies out of the house that don't include you; And I can go on and on.
Men will always cheat even if they don't do it physically they do it mentally by thinking on that hot young intern at the office while you and him are making love. Trust me I know.
I am a man.