Corrective gels on flash. When, how?

I'm finally starting to gel my on-camera flash to match the existing lighting, and I love the improvement I see in my images!  By now I can usually recognize when I need an orange gel for tungsten, and a bluish gel for cloudy.  Basically, though, I'm just eyeballing it.  If the scene looks blue, I add the blue gel; if it looks orange, I add the orange gel.  (And if it looks very blue, I add the very blue gel, etc)  Anyway, I have two questions related to this.

Question #1.  Is there a more objective way to determine what color of gel to use in a given situation?  For example, is there a practical way for a photojournalist in the field to "meter" the color temperature of the ambient light?  Sometimes I can't tell for sure if the dominant indoor lighting is tungsten, fluorescent, or daylight-balanced fluorescent.  And sometimes in mixed lighting it's hard to tell which is the most dominant source.  In short, how do you guys decide when to use a corrective gel and which gel to use?

Question #2.  When I set my camera to AWB, what is it actually doing?  I mean, I know the result of it, but what is it doing from a technical standpoint?  Is it just reading all the colors and averaging them out to a neutral gray?  If so, does it use spot metering, evaluative, or what?

Thanks a lot!