Turn the spindle by hand, rotate the spindle back and forth. If you feel any roughtness or don't feel right, walk away! It should be smooth as silk. The rest of the grinder will depend on how hard of a life it has had. Wear on the slides is important too. The Doall should have rollers on the table travel, again roll it back and forth, again, should be smooth. Of course, you probably can't check this if it's a hydraulic operated one.
Oh, if the grinder is under power and has a wheel on it, surface grind a piece of metal and see what kind of finish you get. IF under power and the spindle has a horrible wine noise to the spindle, walk away, unless you want to spend $1K plus on a DIY spindle rebuild. Overall, DoAll made/sold nice grinding equipment back in time.