That is not actually true, because YouTube is quite CPU intensive. If you perform the experiment you just proposed, you will find YouTube has used a significant portion of the battery. But I see what you mean.

You could also try this: if Google Now is waking your phone up any significant portion of the time, it'll show up in the list as a low percentage app. You can click on it to see how much time it's keeping the CPU awake, keeping the screen on, using the GPS, etc. I believe Google Now processing is accounted to the Google Search app (because that is where all its output shows up). On my phone, which has Google Now enabled, it shows zero active time for GPS, screen awake, CPU, etc.