During a lane change, at which points should a driver check mirrors, according to the manual?

49 CFR 383.111(a)(7); AAMVA CDL Manual Section 2.4.2, pp.2-10 to 2-11

Checking mirrors at just one point in a lane change isn't enough, because traffic conditions change in the seconds it takes to complete the maneuver. Checking before, during, and after the change catches a vehicle that moves into your blind spot exactly when you signaled. This sequence is specific, testable material on the knowledge exam.