What gauge is this extension cord

Sure, measure one wire with an Ohm meter and check a table like this:

If you've got an accurate ohmmeter, you can do as the above. Otherwise, go to a hardware store and look at what they've got that's equally fat with the same sort of insulation That may give you some indication of the wire gauge.

The cord type and gauge should be embrossed or printed along the jacket, about every foot or two (I don't remember the minimum distance)

Bob

I'm assuming you've checked along the length of the insulation to see if it's printed/stamped on that... yes?

Great charts, especially the first one. I should be able to get a decent reading. I'll check what the old one is too. Thanks.

Measuring the resistance of the 100-ft cord would give you the following example readings:

Unless you have a milli-ohm meter, you might have a hard time getting an accurate reading. Something else to try would be to measure the voltage drop by connecting a 1500W space heater to it. The approximate voltage drop based on the formula V=I x R would be: