We propose a new observable for the 21 cm global signal during the dark ages, "the dark age consistency ratio," which could serve as a critical test of models beyond the standard Λ cold dark matter (ΛCDM) model, particularly in future missions using a telescope on the moon or satellite orbiting around the moon. The new observable is motivated from the fact that the shape of the functional form of the 21 cm signal against the frequency is almost cosmological-parameter independent in the ΛCDM. The dark age consistency ratio takes a certain definite value in the ΛCDM case, which enables us to easily falsify models beyond the standard model motivated by some particle physics and gravity theories. The new observable just needs measurements of the brightness temperature at a few frequency bands during the dark ages, and thus it allows us to test cosmological scenarios even with limited information on the 21 cm global signal at the early stage of future lunar missions.