My brother's wife is cheating. How can I get proof?

I recently had to go an hour out of town..close to where my brother and his family live. I was passing by a small restuarant..and saw her and another man standing together and he quickly kissed her on the cheek and walked to his car and she got into hers.

My brother is out west working on a oil rig temporarely to help them catch up bills. She has no idea what I saw or that I was even going to be in her area...I cant afford to go there and sit for days and watch...so whats the next best thing?? Do those spy phone softwares work? Serious help needed..