How do I test an extension cord with a multimeter?

Set your multimeter to continuity or the lowest ohms range. Unplug the cord completely. Check continuity by touching probes to hot, neutral, and ground ends — you should get a beep or near-zero ohms on each. Then check for shorts by testing hot-to-neutral, hot-to-ground, and neutral-to-ground — any beep means a short. If all continuity passes and no shorts exist, the cord is fine; otherwise, recycle it.