HELP! NEED ANSWERS! It's In My Hands!?

wow this one id kinda hard but i think i would tell him anyway or maybe i would set her up like try and find out if she is doing stuff with this other dude like going on dates and stuff and if so since you and your ex are cool then he would'nt mind hanging with you for example ifthe new girl is going to the movies with the person she is seeing then you find out when and where then ask your ex if he would like to hang with you and go to the movies and then oops! what is she doing here with him(he sees it all and there is no explaining to do on your part)