I would push fairly hard on the belt with my thumb on the largest open free run of the belt (which is awkward to do). Deflection should not exceed 1/2". A/C belts on 60 series are skinny and seem the most likely to get loose (in my experience). How old is the belt? Shine a flashlight on the belt and look for shinyness or rotting/cracks on the back side. If you see this, your belt is suspect.
Also, you are in TX, so the belt sees a ton of heat. At some point, it might be easiest to get a quality new belt (Bando, Mitsuboshi, Gates) from Rock Auto or your dealer, and install that. Remember that anything involving the belt drive on a 60 series is a giant pain, as far as access, etc. If you need to pull the radiator for some reason, or do all other belts, you might as well do the whole stinking mess at one time. While the belts are off, spin all tensioners for smooth operation, and check that they don't have excess wobble (bearing runout). All our trucks are ancient/high miles, so all items should be viewed with suspicion.