Pyroxenes are usually identified in the field by their nearly square, 87°/93° cleavage, as opposed to the similar amphiboles with their 56°/124° cleavage.
Geologists with lab equipment find the pyroxenes rich in information about a rock's history. In the field, usually the most you can do is note dark-green or black minerals with Mohs hardness of 5 or 6 and two good cleavages at right angles and call it "pyroxene." The square cleavage is the main way to tell pyroxenes from amphiboles; pyroxenes also form stubbier crystals.
The last two pages in this gallery are classification diagrams for the pyroxenes.