I have a 100' extension cord from the junk, and I wondered if there is
a way to tell what gauge the wire is. The ends are molded on so I
can't see the wire.

I only know that it is a lot heavier than my current 100' extension
cord, but I don't know what gauge that one is. Also, sometimes they
make things thicker and maybe heavier just by adding more
rubber/vinyl.

And it has W-8 embossed on one end.
I searched on W-8 'electric cord' and got two good hits but they
were just about cords being recalled, and they said they cost between
6 and 90 dollars depending on length and gauge. So I guess W-8
doesn't tell me anything.

Is there any way to know?