Check your water bill. If there is no sewer charge, you are on septic. Or check the property: if you have a green/cap-style access lid in the yard, that is a septic tank. Most rural and exurban Columbus 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 district maintains records of permitted septic installations searchable by address. The city of Columbus itself is almost entirely on municipal sewer; septic systems concentrate in the exurban townships of Delaware, Licking, Fairfield, Union, Madison, and Pickaway counties, plus the rural edges of Franklin County. Most homes built after 1990 in unincorporated Delaware County, rural Licking County, and Fairfield County are on septic; municipal sewer expansion into the outer townships has been gradual but slow. Columbus conversion-grant tracking: Delaware Public Health District, Licking County Health Department, and Fairfield Department of Health periodically coordinate with regional sewer authorities on conversion programs that subsidize the connection cost for properties within 200 feet of an existing main. Programs come and go; check the relevant county health district website annually if conversion is on the table. The grant typically covers 25-60% of total project cost.