How can I test an extension cord for internal breaks without a multimeter?

Plug a lamp into the extension cord, turn it on, and slowly bend the cord along its length — especially near the ends and kinked spots. If the lamp flickers or goes out at a certain spot, you’ve found the internal break. No flicker means the cord is probably fine. If it flickers, replace the cord — a broken internal wire can overheat or arc, and it’s not worth the risk.