You can't reliably tell without cutting the cord.

You can’t. Unmarked cords are a guessing game, and 14 vs 16 gauge is close enough that eyeballing it won’t cut it for safety.

Thickness is the only visual clue, but it’s not a sure thing. A 14 gauge cord is slightly thicker and stiffer than a 16 gauge, but manufacturers use different insulation, so a beefy 16 gauge jacket can look identical to a thin 14 gauge. Weighing it is equally unreliable—rubber vs vinyl insulation throws off the feel.

If you’re plugging in a space heater, a window AC, or anything that draws 10-15 amps, do not guess. The difference between 14 and 16 gauge at that load is the difference between “works fine” and “melting cord.” Cut a small section to read the wire diameter, or just buy a new cord with clear markings. It’s ten bucks.

You want to be sure, not sorry.