In order for someone to cheat, they have to have the time to cheat.
They would have to be in an environment that allows them to meet in the first place, if it's someone new.
The work environment and their personal social circle are the most common places to have exposure to the opposite sex for example.
The Work Environment
Let's first take a look at the work environment. Is your partner in an occupation where their exposed to the opposite sex ? For example, if a sales men worked in a retail environment he would come in contact with a lot of women as opposed to a truck driver that's on the road all day. Another example would be, a women working as a waitress in a bar would come into contact with a lot of men. So consider carefully there occupation and daily routine. Don't forget to consider where the crowd at work goes for lunch or maybe some go to a bar after work.
Personal Social Circle
Your partner has a history with the friends in their personal social circle, like best friends have secrets. The question here is, does your partner have an opportunity to cheat and if so would it be with someone through work or their personal social circle ?Not only as a requirement
The 5 W's
Who, would your partner be cheating with, a friend,
a co-worker, an ex ?
What, is going on ? Cruising the bars, strippers, prostitution, an affair, friends with benefits ?
Where would this be happening ? Hotels, motels, the car, their place ?
When, would this be happening ?
(Check your timeline)
Why, would this be happening ? Not satisfied in the relationship. emotionally or physically ?
(Check your Motive)