Ideally, someone who goes to evaluate dogs at the shelter should be very knowledgeable about dogs, and especially their body language. There are quite a lot of books out there on the subject and that's where I would start.
Personally, I think you should also know and understand the breed of dog you're going to evaluate.
There's an article on a Shiba website that outlines a very, very basic evaluation of dogs at the shelter to see whether they might be candidates for rescue, and maybe that'll be interesting / helpful to you:
http://www.shibaweb.com/rtemp.htm