Since you want to measure it using an ADC which will most likely be connected to something else, you need to electrically isolate the high voltage from the low voltage.
You can do this using high voltage capacitors in a similar voltage divider network to a resistor voltage divider network.
You'll need to calculate how much current you'll need and the frequency of the Ac source to determine the actual uF values using non-polarised caps. Then, pull off and rectify the output.
Try some experimentation with this setup.