How do you tell which gauge extension cord you need?

You must first figure out how much of a load you are planning for.

This is done by reading the owners manual for that product or often times it will be listed on the product itself in the way of amps.

14 gauge wire will handle up to 15 amps. 12 gauge 20 amps, 10 gauge 30 and so on.

Length of run is also a factor . The longer the run the more ampreage drop you will have.

Hope this helps