You can feel it. Instinct.
There is no way to know what happened in the past. You could ask her but she could lie -or- you could destroy your marriage when she finds out that you do not trust her or her loyalty to you. Do not dwell on the past. What is over and done is over and done. Think of the future, and the life you can have with your wife if you do not allow jealousy to ruin it. Even if she did have an affair in the past, if it is over, it is over by her choosing and she has chosen you. Look forward with love and if need be - forgiveness. Worry and doubt will not add one second of happiness to your marriage but it could end it. Are you prepared for that? Do you love the woman enough to let the past go?
I believe its been going on with different men our whole marriage . As I think back , also I believe it has happened recently