Sawyer Water Filtration System - How to know it's working?

[Arizona4x4]

I just picked up one of these filters. The reviews seem pretty good on it. Now that I have it, it's small! But what I want to know is how do you know the damn thing has filtered enough of the bad out? Better yet, how jacked up can the water be that you can filter through this thing? Does it matter if the water is from a warm source, like a pond which likely has bacteria growing comfortably in the water or does it need to be from colder source like a stream?