The service manual says to change oil every 150 hours, 6 months or 1500 miles. With this info I would say if a rhino has more than 300 hours per year that would be high milage.
Just look at the underside of the rhino for tears in the metal, checkout the suspension for bends or cracks, check out the aftermarket wiring for correct placement and protection, look at the oil to see if its clean or not, check diff fluids, tire wear, brake pads, does the 4x4 engage, then drive it around a little.
It's hard to find a lot of manufacturer issues with a rhino, not much goes wrong with them, now the rzr 800 that is another story. Been there and replaced most of my rzr.