Courtesy of James K., Virtually Green, San Francisco
If you are considering buying a used tiller, ask to take it for a spin for an hour where you can till some deep soil.
Tighten up all the bolts you can find and then check to be sure they're not loose after the hour of tilling.
Test all the accessories to make sure they work.
Look for any oil or fuel leaks at seals and tubing.
Check to be sure the engine exhaust is clean.
Inspect the tires.
Listen for any odd noises, especially squeals or grinding sounds.