How do I test an extension cord for internal breaks?

Use a multimeter set to continuity or low resistance. Test each conductor end-to-end — infinite resistance or no beep means a broken wire; >1 ohm for a 6-foot cord means corrosion or partial break. The old lamp trick (wiggle cord with a light plugged in) only finds obvious breaks, not high resistance. If you’re unsure, just replace the cord — a new one costs under $20.