There has been some confusion on bugguide when it comes to telling the two widespread species in the Eastern Nearctic apart, S. niger and S. petrophya. Horn 1881 provides a key and figures the latter species, but this figure is a bit misleading in its depiction of the typical pronotal shape.

Sandalus petrophya can be summarized as having a relatively transverse pronotum compared to the more elongated S. niger (i.e. rectangular versus square, though both are trapezoidal). The lateral margins of S. petrophya are posteriorly either parallel or slightly constricted (resulting in a noticeable angle along the sides), whereas in S. niger they form a mostly straight profile and never appear to have an angulate margin.

Coloration is somewhat informative, but not fully reliable. S. petrophya is often a light brown, while S. niger is typically darker. Think of a cup of coffee before and after adding cream, with S. niger being the darker of the two. Males of S. niger are, however, quite variable. Some are solidly dark, but a majority are only dark on the head and thorax, having very lightly colored elytra. Females of S. niger are always dark throughout, while females of S. petrophya are consistently lighter.

Size is helpful when comparing a series of specimens, but unreliable on its own. Females are larger in both species, and females of S. niger are often much larger than those of S. petrophya, but there is overlap with smaller specimens. The largest S. petrophya are around 18mm, while those of S. niger can get nearer to 24mm. Males of both species are similar in size, though S. niger probably averages a tiny bit larger. The smallest S. petrophya (like the one seen here on the right) seem to be smaller than the smallest specimens of S. niger.

There appears to be a clear difference in the flight periodicity for these two. S. petrophya has been collected from late June to early August, while S. niger is reported from late August to early November. Likely, these species make use of different host cicadas, but essentially nothing is known about what these might be.