What is an indicator of non-condensable gases in a system?

The concept behind this question

Evacuation pulls air and moisture out of a system before charging, and it matters because both cause real damage. Moisture combines with refrigerant and oil to form acids that corrode the system from the inside, and it can freeze at the metering device. Non-condensables raise head pressure and cut capacity. A deep vacuum lowers water's boiling point enough to boil it off at ambient temperature, which is why a micron gauge — not the compound gauge on your manifold — is the instrument that tells you whether the job is done.