the SDA line is pulled high with a 2k to 10k resistor normally. so when the i2c is idle, measuring the SDA line pin with a 100k resistor, and 100k to gnd will put the adc within a range to see up and down voltage changes. you can use gnd, or a output pin put to gnd, you will need to verify voltage with a volt meter across gnd of 100k resistor and adc pin to adjust gain to see what actual voltage is.