When performing the "check air flows to all trailers" test on a set of doubles, what confirms that the entire system is properly charged?

49 CFR 383.115(e); AAMVA CDL Manual Section 7.4.1, "Additional Air Brake Checks," p. 7-4

This test directly catches a closed shut-off valve somewhere in the chain of trailers, a defect that would otherwise leave rear trailer brakes completely inoperative with no dashboard warning. It's listed as one of the manual's own "Test Your Knowledge" questions for this section.