Check your water bill. If there’s no sewer charge, you’re on septic. Or check the property: if you have a green/cap-style access lid in the yard, that’s a septic tank. Most rural Cincinnati and exurban properties are septic.
More detail
Three quick ways to confirm. (1) Water-and-sewer bill: municipal sewer customers pay a monthly sewer charge typically equal to or greater than the water charge; septic customers pay water only. The bill line item will say "Sewer Service" or similar. (2) Yard inspection: a green or black plastic riser lid (10-24 inch diameter) flush with grade indicates a septic tank below; a metal grate or concrete cleanout cap indicates municipal sewer. (3) County records: each county health department maintains records of permitted septic installations searchable by address. Roughly 60% of Greater Cincinnati housing on city water is also on city sewer; the remaining 40% (mostly older exurban single-family) is on septic. Most homes built after 1990 in unincorporated Hamilton County, rural Warren County, and Clermont County are on septic; municipal sewer expansion has been gradual but slow. Cincinnati conversion-grant tracking: Hamilton County Public Health, Warren County Health District, and Butler County General Health periodically run sewer-conversion grant programs that subsidize the connection cost for properties within 200 feet of an existing main. Programs come and go; check the relevant county health department website annually if conversion is on the table. The grant typically covers 25-60% of total project cost.