I often scout with an early probe - 9 or 12, but although I recognise how important it is, I don't actually know how to tell how much Chronoboost is being saved by my opponent.
The simple answer is to click on the Nexus. You can see the energy they have. If you have health bars on, you don't even have to click on it at all.
Instead of clicking on the Nexus - press 'Alt' button. In 'default' case you will see all units/buildings health/energy bar. That is more useful.