The white sign is simply the first faint outline of the second exo-skeleton or new skin forming underneath the old as molting approaches. It is very hard for laymen to distinguish. It usually appears about two weeks prior to molting. This marks the appearance of the new shell underneath its present hard shell. Red Sign - Hard crab which will molt in less than two days.
Amateurs can usually detect the red sign, but with considerable practice.
Rank Peeler - Hard crab which will molt in a matter of hours. Simply examine her abdomen and note the color of her apron. A red-sign Sally will have a dark, purple colored, abdomen. See images below. The reason this occurs is very apparent when you actually see the female busting out of her old shell.