Ripple factor detection and measurement
You need to define what you want to measure better if the answer is to be sure of meeting your need. 

You may define ripple factor as eg


Fr = (Vmax-VMin) / Vmax


eg if the supply is 300V max and 270V at the bottom of the ripple then 


Fr = (300 - 270) / 300 =  30 / 300 = 0.1 = 10%


How to do this acceptably depend on acceptable accuracy and what is meant by automatic.

(1) Automatic, easy, minor technology.

Micro controller with ADC.
 Resistor divider to scale Vinj to within ADC range at all tikes.
 ADC samples continually and determine Vmax and Vmin
 Vmax - Vmin determine = Vripple
 Fr = Vripple / Vmax. 

(2) Minimum effort. 

"Automatic determination of Voltages".
 What is done with them depends on requirement.
 Equipment = 2 x capacitor, 2 x diodes, a few resistors.

eg Vmax can be determined by using a peak detector and an RC filter + a diode. - Vin is passed via a diode and resistor to a capacitor. A much larger resistor is connected from capacitor to ground. (eg diode via 1k to 100 uF. 1M to ground).
 Vcapacitor ~= Vpeak - Vdiode.  

Vmin can be determined using a minimum voltage detector.
 Resistor from some Voltage >= Vmax to a capacitor to ground.
 Diode from Vin to a diode cathode to capacitor. 

Capacitor voltage will be Vmin +Vdiode.  

This give Vmax and Vmin which can be measured on analog meters or whatever and scaled as required.