As to letting him know that you know he has contacted his ex etc, that's up to you. If it teases your mind you could ask him if he has ever had desires to contact his ex. If he says no then watch for his guilt oozing through. If he says "oh yeh, in fact I message her occasionally, nothing to it though" then you're on a winner.
But humans are not perfect and over the years I've found that some people in relationships expect perfection. I personally think there is nothing wrong with keeping in contact with an ex....providing there are boundaries, trust in their new one and its based on friendship...especially when there is children involved. Its the mature thing to do, act like adults.
If the contact between my wife and her ex was secretive and there were vital bits of information missing (eg she married me) then I'd be suspicious and granted, I'd keep an eye on the messages if I could, to see if they were meeting secretly. If she did meet with him and the time and date of the meet was known I'd let them meet then ask her where she was that day or during those missing hours. If there was blatant lies then it would be over unless there is good reason.
Be careful though. I knew a couple that went through this. Turned out the husband had a grand friendship with his ex's father, a strong mateship. He knew his new wife would be upset if she found out there was any communication going in that family so he kept it secret. The old man was dying of cancer and had weeks to live.
So be careful and don't be rash. Benefit of the doubt is part of giving faith...until it is proven otherwise.