Oh yeah the fuel quality can vary... worst fuel I've ever experienced must have been in Kansas. In fact I raced on that crap during the '03 Nationals!
The midwest has equally crappy gasoline too - not just diesel. I have no idea why the fuel quality in that particular region is so poor...
As for tips to avoid the bad diesel... what I do when I'm traveling cross-country in foreign lands, is generally speaking, try and fill up at the newest, cleanest, best name-brand fueling stations. There are no guarantees however. When fueling up locally, I get it from places that I know from previous experience to have good fuel.