Well the laws of physics demand that it either be leaking or burning---there's no other explanation for oil loss.
If it were burning a quart every 500 miles I'd say you'd be seeing smoke out the tailpipe. All I can suggest that doesn't cost $$$ is a) have a friend follow your car as you go DOWN a hill, in gear, and let off the gas and let the car just slow down, in gear----then ask your friend if he say blue smoke.
Also you can get the car on a lift and look for a pretty good size leak--sometimes they are hard to spot.
If nothing obvious shows up, then have a "cylinder leakdown test" done on the engine---it'll cost maybe $100 bucks but it'll tell you for sure what the internal condition of your engine is.