There is described a method of operating a concrete mixer truck having a rotary drum configured to receive fresh concrete and an on-board computer. The method generally having: performing an unloading operation including unloading a first portion of the fresh concrete from the drum at a job site, a second portion of the fresh concrete remaining in the drum as return concrete subsequently to the unloading operation; the computer acquiring a first signal indicative of a presence of fresh concrete in the drum; the computer acquiring a second signal indicative of termination of the unload operation; the computer activating an indicator perceivable by a driver of the concrete mixer truck contingent upon receiving both the first signal and the second signal, the indicator signalling a presence of return concrete in the rotary drum to the driver.