follow him and make sure what's he doing during the day and when he is out. when u can check his phone( messages, missedcalls and calls...) then if he was cheating dont panic, first take some photos to have an evidence not only one, take a lot of pictures in different days and then post them to him( ur home but give it to him to open the mail because its for him) by the way have those photos with yourself too. and see his reaction about than mail. ask him about it and if he doesnt say anything about it, say that u know what it is and u know what he is doing.
good luck