The easiest way to keep the breeds straight is to forget the King Charles' portion at first and focus on Charlie versus Cavalier. The rest will fall into place. Charlie ears should typically be set low to accentuate the domed head and Cavalier ears should be set high to accentuate the flat appearance of the skull. Note, however, that some Cavaliers have lower earsets than preferred and some Charlies have higher earsets than preferred, so focusing on the roundness of the head and nose placement will probably help the most in this comparison. Charlie tails are typically docked and Cavalier tails are typically undocked and left long.
Cavaliers are also slightly larger.
Beyond that both breeds come in the exact same colors, which share similar but not identical names. Charlies come in four colors. The parti-colored dogs carry white in their coats. Blenheims have red patches on a white background. Occasionally one finds the famous Blenheim spot on the top of the skull resting between the red that usually covers the eyes and ears.