Loss of Sex Drive
Unless there is another cause of stress (i.e., difficult assignment at work), then a sudden loss of interest in sex may indicate your loved one is getting it elsewhere.
Defensiveness
A cheating partner often feels bad deep down and this guilt can come out via defensiveness or blame placed on you. Be on the look out for signs of aggressiveness from your partner.
Host of Excuses
They may seem cliche, but comments about needing to work late, having to take a business trip, or needing to unexpectedly meet with a client, can all be signs of cheating.
Telephone Hang-Ups
If your partner's new liaison is bold enough to call your home number, he or she may hang up if you answer, instead of your partner. Be on the look out for frequent hang-ups.
Gifts Out of Nowhere
Besides defensiveness, your cheating partner may try to throw you off track by being extra nice. Flowers, chocolate, and other gifts, especially if they are typically uncommon, could be a sign of infidelity.
Please do keep in mind that none of these signs are guarantees of a cheating partner. They are only indicators, and the more of them you see, the higher the chances something is going on. Communicate with your partner if you feel something is going on, and you'll give your relationship the best shot at surviving over the long haul.