Clicking the Simulation button will allow you to test your Physics Asset.
The downward triangle next to the Simulation button will give you access to Real Simulation and No Gravity:
Real Simulation - Simulates the entire Physics Asset with gravity enabled.
No Gravity - Simulates the entire Physics Asset but does not turn on gravity, leaving you to ctrl+click to poke the Physics Bodies in a zero gravity environment.
This is useful for finding any interpenetrating Physics Bodies or Limited Physics Constraints already outside of their limits.
You can also simulate a chain of joints by toggling Selected Simulation on, then clicking the Simulation button.
Selected Simulation - Does mostly what it sounds like, it only simulates the Physics Bodies you have selected (using ctrl+left click you can select more than one
Physics Body) and those down the hierarchy from the selected Physics Bodies. So if you select the shoulder, the entire arm will simulate.