Just my thoughts but why not, instead of confronting her with proof so she feels backed into a corner, asking her upfront for an honest conversation and then sitting down calmly saying you will trust her response and you won't get angry but has she cheated on you.
I think if you want honesty from her you have to be honest yourself and snooping isn't being honest. I just feel this has less of a chance into escalating into something else where you both misunderstand each other. She isn't going to be open and receptive if you sit there and lay evidence in front of her. After all when you asked about the pregnancy tests etc you got an honest answer and you felt better and that worked. Why not try it again before snooping and coming to your own conclusions.
