Remove plastic slipcovers from the couch and see if it still squeaks. Slipcovers can cause squeaks, especially if they are bunched around the couch's cushions.
Remove all cushions from the couch. Push down on the couch's springs or supports to try to isolate the source of the squeak so it can be fixed.
Check the couch's frame for loose bolts and screws. Tighten all loose bolts or screws you find. Loose fasteners in the couch frame can cause squeaks, but, more importantly, they can be a safety hazard.
Tip the couch over to access its underside. Locate the coil springs on the underside, and spray them with a spray lubricant. Turn the couch back over, and test it for squeaks.