Check the launch of the module with the option turned on and look at the fingerprints on this page:
https://browserleaks.com/webgl
For example, I ran the module (GeckoFx45) several times, and the prints are always the same:
| WebGL Report Hash | 2742A3D40FE1B394957271025ED226DB |
| WebGL Image Hash | 8734B882545C11F10101E156003CDD57 |
