Sorry you feel like this.
I would like to say that I have seen many people on here sit down and ask their DP/DH outright if they are seeing someone else/been/being unfaithful and when they get the answer they want - NO, they still come away either then or at some point in the future, still feeling unsure.
Yes in an ideal world a good little chat would clear the air and resolve all. Sadly, if someone is devious enough to cheat then they are devious enough to lie and/or do whatever they need to do to cover their tracks and perhaps continue.
Sadly, once they know you have an inkling, future "affair/cheating" behaviour is modified leaving it harder for you to know.
Something has made you think like this and you need to decide how far you are willing to go to find out more information. It could be simple things not involving snooping but perhaps asking where he is working late and turning up to surprise him with a coffee/small treat snack or tea, checking if he where he said he is. If he isnt then this opens up a conversation allowing you to question where he was (without alerting him to the fact you think AFFAIR) and judging his reaction.
You could keep an eye on mileage on the car and see if there is any unexplained depending how regular his lifestyle habits are. Have a look at bank statements (if you have joint accounts) to see where cash is being withdrawn, cards being used etc. Its subtle snooping without invading his privacy. You could also now start keeping a record of all his working late and gently mentioning as he has worked X hours overtime this week and last week, shall we do X or why don't you treat yourself to X(something he wants/has had an eye on) with the overtime money - then perhaps after payday - "ooh thats strange, did you not get your overtime paid this month?" - Gently, non accusing but his reactions may put your mind at ease or provoke your current line of thought, meaning you need to decide how to proceed further.
Or you could just go straight in there with something more instrusive like checking his computer history, mobile phone,pockets, searching cars etc
Proceed how you feel comfortable, bearing in mind how you will explain yourself if hopefully you are barking up the wrong tree.