Although Evirus mentioned it off-hand, the problem was quite serious for me: at first glance, Ami and Sumire look exactly the same. All the usual clues, such as the shape of the eye, eyebrows, general hair style, and eye color, look the same. Still, it's easy to tell them apart if you know where to look.
The most important clue is the "Naru Narusegawa hair line" (sans the antennas): the cutesy rustic roofline which Ami carries and Sumire lacks. Do not focus on minute differences in the hanging bangs, they are too vague. The roofline is easy to recognize quickly.
Note that this clue works well with medium zoom models.
The alternative clue is the eye color versus the hair color. Although it is true that Sumire's hair color is a darker hue than Ami's, the difference is only useful when they are together in the frame. Without each other as a reference, it's impossible to grade the hue. The solution is to use the eye as a reference. Sumire's eye is similar to her hair:
Ami's eye is different from her hair:
The backup clue is especially useful when the girls get wet and the hair style changes (e.g. ep.06).