Traditionally geographical, anywhere north/ north west of the city centre will be Wednesday, anywhere south/south east of the city centre will be United.
Parson Cross, Hillsborough, High Green, Firth Park, Wadsley Bridge, Upperthorpe, Stannington all strong Wednesday areas.
Manor, Arbouthorne, Woodseats, Gleadless, Dore + Totley, Sharrow, Ranmoor mostly United.
However, you'll find loyal contingents to both clubs in almost every district. I'm aware of a Manor Park + Arbouthorne Wednesday supporters group, including myself!