measure dc voltage after wall xfmer= transformer and usb cable.
May use a usb to headphone adapter to easier place probes
an analog meter uses electricity from the electrical outlet past the xfmer box that contains 2 coils; winding of different number formerly used to drop 120 v ac from outlet to about 20v ac that is rectified passed through one way door to positive diodes and capacitively filtered to the precise voltage written on laptop pay attention to polarity of i/p if buying a replacement universal supply with multiple tips . This is the theory which could now be done perhaps with op amp internal or smaller integrated cct with feedback. ..
Basically output of plug adapter should be about about 19Vdc and the same at the usb cable o/p to laptop