Client supplied Nest doorbell cam. It is hard wired with a power supply. They are using this aftermarket chime:
[Link: wasserstein-home.com]

I need to determine if the chime or the Nest doorcam is malfunctioning.

The Nest doorbell has 2 terminals for power - 16 volts AC is present. Camera/talkback functions properly. When the doorbell button is pressed, what should I see at the terminals? How do I test? Contact closure? Voltage change? What do I test for while AC voltage is already there?
Thanks