By looking? Really, water is only there to help moderate the temps. If your temps are stable, fuggedaboutit.
When I used to use water (and that was years ago) I never needed to add (on a 6-7 hr smoke).
Here's my detailed article on using water in the water pan: http://virtualweberbullet.com/using-a-water-pan-in-weber-smokers.html
Assuming you start with a full pan of water, you should check every two hours by opening the access door and taking a peek at the water level. If you're cooking at night, you'll need a flashlight. Keep in mind that the pan will be catching grease during the cook, so what you see in the pan is a combination of water with grease floating on top. If the level looks low, add some water using any of the containers mentioned in the article. But don't fill to the top, you want to leave room for the pan to continue to catch grease.
As a general rule, I use water whenever I'm cooking low & slow using the Minion Method. Sometimes I need to add water, sometimes I don't, it just depends. Not only does the water help with temperature control, but the steam creates a moist environment that helps with smoke ring formation.