Calcite (CaCO3) is easily identifiable in a number of ways. Its softness is distinctive; at a hardness of three, it scratches easily. It famously bubbles when you put a drop of acid on it, though I have only rarely carried acid with me for testing. The places and minerological environments in which it occurs are broad, but can be helpful. Its color is highly variable, but tends to be on the white/pastel end of things. Its rhombohedral cleavage is very distinct, and frequently a dead give-away, as are its variety of crystal forms. But the photo above shows the feature that, in practice, is the one that is most often the visual cue that brings up my hammer for a quick scratch test for confirmation: striated weathering. My understanding is that this is related to the mineral's perfect cleavage and relatively high solubility in water. Cleavage is the result of planar arrangements of atoms that are more strongly bonded within planes, but less strongly bonded across planes. As a result, between those planes is the easiest place for the mineral to split. In the case of calcite, this means it's also the easiest place for its ions to go into solution. So if calcite grains of a size large enough to be visible have been sitting out in the weather for a significant amount of time, they will generally show the linear etching- striations- seen in the sample above.