Refrigerator monitoring using ESP8266
While a voltage sensor is theoretically feasible with few diodes and a resistors is not something that i suggest ad all when 230VAC is involved.

If you don't want to "hack" the refrigerator you can exploit its "power-on" led light, if any, with a photoresistor/phototransistor. Or you can actually hack the refrigerator and put your optoisolator sensing this power-on led. If you don't have any led you can use an hall effect current sensor, for instance you can search on google "wcs1500 module".

230VAC is dangerous so i'd go with the first two options, if possible.