The males turn very dark for the spawn. Red tails are less noticeable on crappies, IMO. All white crappies have the vertical stripes (even the dark males), black crappies don't have stripes, they are more speckled. Males of both species turn black for spawn.
It's my understanding that the egg sacs will have more and more small bright red blood veins, the closer the female is to spawning. They will get sort of mushy and off-yellow if she has gone too long without getting to spawn and she is re-absorbing the eggs.