Common Ravens can be told from American Crows by a couple of things. The size difference, which is huge, is only useful with something else around to compare them with. Ravens are as big as Red-tailed Hawks, and crows are, well, crow sized. The wedge-shaped tail of the raven is a good character, if you can see it well. Crows sometimes show an apparent wedge shape to the tail, but almost never when it is fanned as the bird soars or banks (except for a brief time during molt in the summer).
More subtle characters include: ravens soar more than crows. If you see a "crow" soaring for more than a few seconds, check it a second time. Crows never do the somersault in flight that Common Ravens often do. Ravens are longer necked in flight than crows. The larger bill of the raven can be seen in flight, but it is actually less apparent than the long neck. Raven wings are shaped differently than are crow wings, with longer primaries ("fingers") with more slotting between them. The longer primaries make the wings look more bent at the wrist than a crow as the bird flies, and the "hand" portion can look nearly pointed.
American Crows make the familiar "caw-caw," but also have a large repertoire of rattles, clicks, and even clear bell-like notes. However, they never give anything resembling the most common calls of Common Ravens. The most familiar call of a raven is a deep, reverberating croaking or "gronk-gronk." Only occasionally will a raven make a call similar to a crow's "caw" but even then it is so deep as to be fairly easily distinguished from a real crow. Ravens also make a huge variety of different notes. It has been said (attributed to native Americans) that if you hear something in the forest that you cannot identify (assuming you know all the common forest sounds), it is a raven.