The APHA gives some neat posters out with their Information Packets (ive requested a few for summer campers this year) Here's one off of their website: APHA.Com - Paint Horse Patterns
The way I learned it...
Tobianos have color AND white that crosses over their back from the top of their neck to the base of their tail. So, if you are standing at your horses shoulder looking down his back to his butt, and see both color and white over his spine.. its probably a Tobiano!
Overos have one color over
(get it.. overo.. lol im lame :p)their entire back. Usually, these horses are a solid color with white splotches
Tovero is a mixture of Tobiano and Overo. Medicine hat paints are Tovero. Heres the Wikipedia article on them (which im posting because my mare is the horse on the right in the article, LOL) Wikipedia isnt always the most reputable source, but I wanted to post in a hurry =) Tovero - Wikipedia, the free encyclopedia
I hope this helps!