So you have a driver? Like a personal driver? That’s a whole situation right there because that means there’s a lot of time and proximity between your girlfriend and this guy. If he’s driving her places while you’re not there, that’s a lot of opportunity for something to happen.
But suspicion isn’t proof. What exactly is making you think something’s going on? Is she being secretive with her phone? Is she acting different around you? Does the driver act weird when you’re around? Are there actual signs or is it just a gut feeling because you don’t like that she spends time with another man?
You need to talk to her about it directly but without accusing her. Like say “I’ve been feeling uncomfortable about the amount of time you spend with the driver” and see how she reacts. If she gets super defensive or tries to turn it around on you, that’s suspicious. If she’s understanding and willing to set boundaries, maybe there’s nothing to worry about.
But also, if you have a driver and your girlfriend is spending enough alone time with him for you to be suspicious, maybe the arrangement itself needs to change. Can you drive her sometimes? Can you get a different driver? Remove the temptation and the opportunity and see if things settle down. Trust is hard to rebuild once it’s broken so protect it while you still can.