Over the years I have watched my wife interacting with other men enough times that I can generally tell when she is attracted to a guy and when she isn't.
"And I know that thinking about what he and I will be doing will probably get your little willy hard -- so it's fine with me if you indulge in a couple of hand-pussy quickies while I'm out. My sister said she was sure that's what you'd want to do, and it is -- isn't it, hun?"
This actually happened many years ago. One of my wife's co-workers was flirting with her, but in a friendly non-threatening way. She told me about it from the beginning and I told her I was not jealous. I told her she should flirt back, which she thought was a terrible thing for me to say. After a few weeks though she admitted she had indeed starting flirting back, she said it was all just in good fun and she was sure he wasn't serious. However, I was sure that at least he was very serious. He asked her to lunch and I encouraged her to go which she finally did. They were soon doing lunch and coffee on a regular basis. On those days she met with him I could tell she was always extra horny when she got home and we would end up having passionate sex. This was especially true after she met him for drinks after work at a bar near their office. I told her that flirting with her co-worker was great for our sex life and she had to admit it was true. Her flirtatious friendship with her co-worker lasted for close to year until he got transferred to a different city. Although nothing physical really happened between them beyond hugs and kisses it was still hugely exciting to us. I also think it was a big early step in our cuckolding journey.