Bad roots -
rotted: usually blackish, sometimes mushy or slimy, could have bad odor
dead: should be quite easy to see, darker or light in color, brittle, skinny and or wrinkled and really dead looking
damaged: fresh damage, with 'flesh' visible
Good roots look plump, usually whitish color (some plants may have darker color roots), do not feel too soft when you touch them. Often they have lots of fine, hair-like roots.
From what I can tell in your photo, they look pretty healthy (it may be easier to judge by taking close-up photo).
That's what I would be looking for - hope I got it right...Rina