According to Mumsnet:
Five signs your partner might be having an affair
His work habits change - he's out late a lot more than usual, he goes to work at odd times, or he puts in a lot more time than he used to at his job.
He spends a lot more time than usual on his computer and/or mobile phone.
He says he needs some time to be by himself, to "work things out".
He seems to need a lot more privacy than previously - maybe he starts to use a password on the computer, or doesn't leave the mobile phone bill where you can see it.
His behaviour just doesn't add up. There's missing time he can't account for, spending that can't be explained, little discrepancies about where he was and who he was with.
I think at some point since I've been married I've done the first four at least, and I'm definitely not having an affair!