In the SDK there is a hidden function called readvdd33() that returns the internal measured VDD.
000004f8 T readvdd33
On my setup board I am not able to find a correct value since the function always returns 4096. Need to test it on the ESP-01 to see if make any difference.
Update: Other users have reported this function as a working one, maybe their setup was running on batteries ?
Will be nice to have this functions like it is on Arduino !