This is a manual test, since it relies on hardware configurations. It creates two WebGL contexts. It first asks for a context with preferLowPowerToHighPerformance set to true, and then for a context with the default value (false).
Unfortunately there isn't a completely reliable way to know if this test passed. On a system with both an integrated and discrete GPU, assuming that the system is configured to prefer the integrated GPU, and no other apps are running that might cause the discrete GPU to be active, the two renderers should be different. Also, if they are different, then the first one should be the lower power model (i.e. the integrated card).