There are some common types of behaviour which are strongly indicative that your partner is having an affair or simply cheating on you. They are mostly of value combined but even individually they are an indicator that further 'investigation' needs to take place. Those signs are typically the ones below:

1. Cheaters are possessive about their cell phones. They seem to have secretive phone calls; if you enter a room where they are having a conversation over the phone they suddenly hang up or seem to have changed tone or subject abruptly. They leave the room to make suspicious phone calls and they never make the cell phone available for you to check the details of that 'suspicious' call.  If you can, check the list of placed calls and seek for regularly appearing numbers. 

2. If your partner is having an affair he/she may suddenly change his/her appearance and spend a much longer amount of time making sure they look good. If they suddenly buy new clothes or a new perfume (or cologne if they’re male), or if their routine seems to have changed, these may be signs of cheating.

3. Another sign of a possible affair is if your partner’s Internet behaviour suddenly changes. She/he may spend much more time online than usual, countless hours after you've gone to bed, for example; the clue is in the change of behaviour, of course. If you don't live together, this may be harder to determine but you can still see what happens when you stay over your partner's house or the other way round.

4. If your partner’s behaviour towards you changes, such as he/she is now either much more affectionate than usual or more irritable than usual. Sometimes a cheater may pick fights as an excuse to feel better about cheating, as a way to appease their guilt.  Sometimes they may pick a fight so that they can use it as an excuse to leave the house.

The above signs, especially if combined, are a strong indicator that your partner is having an affair or he/she is cheating on you (or that he/she is planning to cheat). However, gather all the evidence before confronting him/her.

What you must NOT do if you have just found out your partner is having an affair:  click here.