Surprisingly, it is possible for a toggle switch to indicate its current state and the state it will change to without having text on the button itself. The problem lies in the fact that “on” and “off” can be both adverbs and adjectives in English. However, buttons that have text outside of their body utilize this ambiguity to their advantage. For example, the trip switch in the iOS design is not clear in indicating whether it is currently on or if it will turn on when clicked. In contrast, the OS X design leaves no room for confusion as the button neither says “on” nor “off” but simply stands by itself. The Windows Metro UI design takes it a step further by highlighting the current state using color and stating “on” or “off” underneath the option text.
https://ux.stackexchange.com/questions/1318/should-a-toggle-button-show-its-current-state-or-the-state-to-which-it-will-chan