you don't,its like that you can never know if she will or wont cheat again all you can do is work at it,
it will take time for you to forgive and forget ,if you can?
the thing is she has betrayed your trust and now you think its going to happen again every time she out of sight,
it may it may not but if you truly love her you want to give her that second chance,well its going to be a long slog for you more than her,
but if you are asking her every time she out what she do and who she meet then you will kill it for you both,
you both need to talk and find out where you want to go and be in this