Take off the belt, and turn the pulleys and listen for a squeak, But this is not 100%, might need a few more rpms to make the squeak.
anything oily will cause a squeak, To remove the oil get some soap, I used some liquid dish soap and while my engine idled i dripped some on the belt, would stop the squeak for a few days, Later I found out it was my belt tensioner bearing that was squeaking, found it odd that soap on the belt would make it stop.
Good luck, nothing worse than driving a squeaking truck