Monitoring Vin on a ESP32

I use this AZDelivery NodeMCU32
(this is a ESP32-WROOM 32)

Where can I find an example sketch that measure directly the Vin (5V) input of the ESP32 using only internals ?

I prefer to can first directly measure the Vin=5V voltage without the use of externals resistors.

But if this is isn't possible or if I have to measure the input voltage before an optional regulator from 3.7V .. 4.2V to 3.3V between the TP4046 and the ESP32, I think to pass via an external diviser.