Visual inspection catches 90% of cord damage.

Set your multimeter to continuity or low ohms (200 Ω range). Test each conductor end-to-end (plug blade to corresponding slot in the female end). You should get near-zero resistance. Then test each conductor to the ground pin — should show open (infinite). That’s it.

If you get high resistance on a conductor (> a few ohms), there’s internal fray or corrosion. If you get continuity between hot and neutral or any conductor to ground, the insulation is compromised. Either way, the cord is unsafe — replace it.

Honestly, most extension cord damage is visible: cuts, kinks, melted plastic, or a plug that feels loose. If the cord looks fine and the multimeter checks out, it’s probably fine. But if you’re unsure, don’t risk a fire. New cords are cheap.