How to set ESP32 RTC clock source

Question

According to the manufacturer's link below, the default RTC timekeeping in the ESP32 is supposedly based on Internal 150 kHz RC oscillator.

Apparently, there is another option called Internal 8.5 MHz oscillator, divided by 256 (~33 kHz) that has better stability without needing an external crystal. That link goes on to say that

To modify the RTC clock source, set CONFIG_RTC_CLK_SRC in the project configuration.

All good information but NO EXAMPLES given. Can anyone point me to an example showing

1. how I can prove what the current clock source is, and
2. how to set actually it?

https://docs.espressif.com/.../system/system_time.html