How do you know what JSON properties a service/entity supports?

Suppose I have a zwave smart plug and I want to control it from Node Red. How do you know what JSON properties it supports when I invoke the "call service node" on it? To me, this needs an API definition per-device so you know all the properties it can accept. This would make it much easier to learn how to change certain aspects of a device from NR.

I am guess this is a HA issue. You may find a better answer on the Home Assistant forum.
But I would look at the docs for HA