Before backing a trailer into a tight space, what does the AAMVA manual recommend as the first step?

AAMVA CDL Manual, Section 6.1.7 "Backing with a Trailer," p. 6-4, Figure 6.5

A curb, pole, parked car, or person behind a trailer can be invisible from the cab before the vehicle ever starts moving. Walking the path first catches hazards that mirrors alone, checked only once backing is underway, might miss.