On an older trailer where it isn't obvious whether ABS is installed, how can a driver check?

AAMVA CDL Manual, Section 6.3.1, p. 6-7

Because many pre-1998 trailers were voluntarily retrofitted with ABS, a driver can't just assume older equipment lacks it. Knowing the physical ECU and wheel-speed-sensor check lets a driver verify ABS presence on any trailer, regardless of its age or whether it has a malfunction lamp.