For most PSUs, look at the label. If they have multiple 12V rails listed there, they're generally multi rail. Your RM850x is single rail only.
Most high end PSUs are single rail (because clueless consumers want that), but some are configurable. Some Corsair PSUs allow you to choose in software, and some be quiet PSUs have a thing to choose single or multi rail.
A decent multi rail PSU is preferred over an equivalent single rail PSU, especially for higher wattage PSUs. The main benefit is that they reduce the danger if something goes wrong