Look for straight running growth rings on the board's face/sides. Doesn't mean you'll find many like that but that's something to shoot for. Look for consistent growth ring thickness as well. Boards that show straight, both on the face and sides, is another indication of little stress in the wood. I prefer 2X2 to work with personally, overall lateral stresses will show up better. But I'll jump right on any nice straight grained board. You kind of get a feel for the wood's density after doing a few but it's a good rule of thumb to kind of weigh one board against one another. That and studying the board's growth ring structure plus it's overall straightness is my critrion.
As far as consistent spine vs diameter, I've had pretty good luck there. But you will get a few from time to time that will show different spine vs their diameter though. -ART B