Need help

So my girlfriend I’m almost 90 percent sure she’s cheating with a coworker, I don’t have proof but I want to scare her, the only idea I have is for someone to text her and say something about people in the office know you’re hooking up with Dave and someone said they are going to message your boyfriend, something along the lines of that if someone has a better idea please tell me. I want to scare her so I can be able to see how she reacts and see if she tells me which will help me conclude whether or not she’s actually cheating.