How can I detect the USB-C power delivery voltage?

You may try the Nirsoft BatteryInfoView:

BatteryInfoView is a small utility for laptops and netbook computers that displays the current status and information about your battery. The displayed battery information includes the battery name, manufacture name, serial number, manufacture date, power state (charging/discharging), current battery capacity, full charged capacity, voltage, charge/discharge rate, and more...

Knowing Nirsoft, this utility shows all the information known about the battery in Windows. Especially, the "charge/discharge rate" while the charger is connected might be the value that you're looking for.

I use a USB-C breakout board and a voltmeter. If you have Windows though, harrymc's answer is easier.