First determine that it is a brome. The spikelets of bromes are large and lanceolate or torpedo-like, containing many overlapping florets. Next look closely to find the fringe of hairs on the lemmas. Finally, you must measure the lengths of the tiny anthers. They are less than two millimeters long.
Almost all the 28 Arizona bromes are somewhat similar! Fortunately, many of them have distinctive features. But Fringed Brome and Richardson's Brome both have the fringe of hairs on the lemmas, so you must then measure the anthers. These are longer on Richardson's Brome, usually over 2 millimeters.