I think it's pretty clear toward that the "off" text paired with the red signifies the current state. Likewise hen turned on. The reason I've chosen this is th

I think it's pretty clear toward that the "off" text paired with the red signifies the current state. Likewise hen turned on. The reason I've chosen this is that the actual button is changing its state (e.g. other toggles merely slide a box left or right with no color or label updates). This ultimately reflects the design principle of giving the user feedback. Clicking the toggle noticeably shows a change in state. There's also some affordance in the design because of the coloring. This isn't perfect (as acknowledged in the comment about color blindness below), but it solves enough of the problem to be obvious. No interface is 100% perfect for every user, every test case, or extreme.