The wide blade of the plug is the common. (Whte wire)
The narrow blade is the power (Black wire.)

If your cord is like a heavy duty lamp cord, flat rectangular shape with a grove running the full length between the conductors that is properly called "Zip cord" because it can be separated like a zipper. Rub the outer covering of each conductor with your finger nail. One wire has a smooth covering while the opposite wire has very small ridges running the full length of the wire. The wire with the ridges is the identified conductor and is supposed to be attached to the common side of the circuit. (White wire side).

Thanks Lazypup. Found the ridges just fine. Curious that extension cords do not have these ridges, or other indication, Although all the writing is on one of the two Zip wires.
As soon as I can get to the store and buy the coorect plug, I will be in business.

while you may have never noticed it before you will find those ridges on all zip cord... lamps, extension cords, appliance cords etc.

The only exception that i have ever noticed is on some of the zip cord that has a clear covering one wire is copper colored and the opposite is a silver color. The silver colored wire is then considered the identified conductor and is to be used as circuit common.

One extention cord has only two ridges. They look like a long line that contiues the entire length of the wire. This was bought in bulk, cut from a larger reel Another extension cord that came in a package with plugs attached has many small ridges, like the original appliance cord. If you hadn't pointed it out, I would never notice.

Another question: I have a couple of very old three-prong extension cords. Probably AWG 16, perhaps larger. However, both prongs on the plug are the same size, so there is no clear indication of polarity. Are these safe to use today?

"Another question: I have a couple of very old three-prong extension cords. Probably AWG 16, perhaps larger. However, both prongs on the plug are the same size, so there is no clear indication of polarity. Are these safe to use today?"

Sure. On a male three prong plug there is no need for a wider blade because you can only insert the plug one way. The plug should already be wired with the white to the common terminal (wide plug). The female end should have a wide slot though, so use of a two prong appliance will be oriented properly. HTH

double check the size of the factory cord as well, it may be 14 guage and if so you cannot use 16g on it. 1500 watts is pretty high for that small a cord.

UL-listed extension cords now must be constructed with #16 gauge or larger wire, or be equipped with integral fuses. The #16 gauge wire is rated to carry 13 amperes (up to 1560 watts), as compared to the formerly-used # 18 gauge cords that were rated for 10 amperes (up to 1200 watts).

I speak American English. I do not know "common" in electricity jargon. I do know positive, neutral, and ground. Please tell me what is "common"?