You don't have to guess at all, the indicators are all there....
On = Light on the stalk is illuminated, and just in case you can't see that there is an arrow on the outside rim of the speedometer.
Active = The words "Cruise Control" illuminated with a reassuring blue glow are shown immediately above the vehicle speed, center of your speedometer..
Off = No light on stalk, no signs on the speedometer.
Those indicators are, of course, true and I do know them. In reality, maintaining an appreciation of the status of the cruise control in a Model S is, in my view, difficult: I don't see the light on the cruise control stalk normally, unless I turn the wheel. In which case, if I'm not using the cruise, it can be annoyingly bright when I get off the highway and turn the wheel and see it - suggesting to turn it off. The arrow on the rim of the speedometer is small and requires specific checking - you can't do this by hand or by feel, and the fact that sometimes the cruise is on and sometimes off, it is a bit of a chore.
Contrast this to e.g. the Audi solution, where the cruise control is always on, unless you push the stalk physically back out of the way - which you will notice immediately when you try to operate it. No need to see any indicators or lights (there are none), you will feel it when you operate it. Also, the cruise on/off button in Tesla is a toggle, so the same button turns on or off depending on the mode it was in. Again, if you get confused by not paying attention to the small visual cues, you may accidentally turn it off when intending to turn it on.
Two more things:
In normal situations the Audi cruise control is ready start driving at any moment - when driving at the speed you want, just press the stalk button to set speed and activate. In a (non-ACC at least) Tesla Model S, you have to press the cruise control to turn it on (including the annoying light you see only occasionally) and then after hopefully you turned it on and not off, to activate and set the speed, you move the lever up or down (which is illogical in itself, why are there two directions to move to activate). If the cruise was not on, you will not feel it by hand, instead you will simply notice that the cruise didn't activate when you let go of the gas pedal - and you have to press a button and move a stalk to repeat. If you get confused and the cruise was already active, this move will actually increase the speed of the vehicle because moving the lever up and down has two separate functions (activate cruise, change cruise speed). In an Audi, there is no harm in re-activating the cruise, it just sets the current speed again because the set-button has a single function, it is not a toggle.
There is another thing: In an Audi, pressing the button always sets cruise speed and activates cruise if it isn't active. This is very useful because if you need to increase speed, you just use the gas pedal and press cruise button again - all the while the cruise remains active and sets the new speed when you press the button. In a Tesla, at least not according to the manual, there isn't a way to set a new current speed without deactivating the cruise control first and then going back through the motions (or using the lever to alter speed setting).
I argue the Tesla cruise control stalk is unnecessarily complex to operate. Operating a cruise is a many-times-a-day operation, it can and should be as simple as driving itself.
In reality, very often I try to activate the Tesla cruise when it is off - or won't bother at all because changing the speed while driving is so bothersome. The moves just don't feel logical at all and there is no "doing by touch".
