You can tell if a tractor has one from a repair manual or from somebody who knows a tractor mechanically. A ORC is part of the tractor, while a slip clutch is part of an implement. The purpose of the ORC is to prevent the inertia of a cutter, running at PTO speed, from driving the TX when the clutch is disengaged. The purpose of a slip clutch is to reduce chances of damage to the PTO drive and implement if it becomes jammed. Incidentally, some people claim that slip clutches are difficult to keep in adjustment, and shear pins are actually more reliable.
Shear pins are soft metal bolts that hold PTO shaft couplers together.
The pins break if too much force is applied to the shaft. Shear pins come in all sorts of sizes, lengths and grades. When buying a used implement that uses shear pins, it's a good idea to determine the proper pin. It's not uncommon to find a regular hard bolt in place of a proper shear pin. I have heard stories too, but have been told that a slip clutch can be added between the mower and the tractor.