There is absolutely no way to tell that the Pronto is "asleep". The pronto definition of asleep is that the processor is stopped, ergo no way to tell since the processor must be running to do/tell anything!
What you can tell is that you were "asleep". To do that you need to use the clock in a known loop to see how long the loop iteration took. You can then make a judgement that the loop took way to long ergo the processor must have stopped, but it is not stopped now!. Also the Pronto never stays "asleepA for more than about 57 seconds. It then wakes up for about 5 and then goes back to sleep. In real terms, since ascribing terms like sleep and awake is just anthromorphising (assigning human characteristics to non human entities), the processor is stopped when it detects nothing is going on and the processor restarts about every 57 seconds or so and then stops again 5-7 seconds later. While the processor is stopped the real time clock is ticking away, so the only way to see that you are in the restart period is by checking the clock while in a repetitive loop like a page loop.
