ok, alot of the above are good. here is my list:
1. change in behaviour. (towards you. on edge. a bit angry. argumentive for the least thing. these are his justifications for cheating)
2. suspicion is one thing. Instinct is another. listen to your instinct/feelings!
3. over protective with mobile phone.(never leaves it unattended gets into a panic if he forgets it)
4. additionl miles on the clock in the car. or the same additional miles done on a specific day.(take note and compare)
5.Suspicious/obvious seamen stains in underware on his retun from a late night.
6.switches off, does not get into conversation, (beause less said the best)!
7. lies! lies about where he,s been. and depending on the person, will lie that they did not met that person. (probably the person they are having the affair with)!