Is it possible to find out which watch faces are always on ones?
In Data Lover you're able to configure in the app settings which extra data to show while in always on mode.
With Simple Lean, I have two options for "always on". The default is the time moves on the screen to prevent burn in. With the second option, the time stays in the same place, but is dimmer - I turn off 1/2 the pixels each time the time is displayed, so no pixel is on for more than 1 minute.
One thing to note is if you take off the watch, the display will turn off with "always on" displays. They use OHR to turn off the display if the watch isn't being worn to preserver battery.