Can I tell if an extension cord is 14 or 16 gauge just by looking at it?

You can’t reliably tell without cutting the cord. Thickness is the only visual clue, but insulation varies, so a 16 gauge can look like a 14 gauge. Weighing it is also unreliable. For high-draw devices like space heaters or window ACs, don’t guess—cut a small section to read the wire diameter, or buy a new cord with clear markings.