That's really a tough question to answer because the sheer amount of different varieties available including hybrids. But if someone was to use a measure to determine the common difference between a Sativa's appearance and an Indica would be similar to the following:
Sativa
Tall, skiiny growing, flufflier, skinnier bud sites, ceriberal high, herbacious to fruity smelling, sometimes minty or spicy, 7-13 finger or leaves, mostly skinny
Indica
Short growing, compact dense, sometimes thicker longer bud sites, stronger body high, mild to strong smoke, sometimes fruity, 7-11 leaves mostly larger or wider