Fuel tank question

I caved in my factory tank a couple of weeks ago, so I am on the hunt for a replacement tank and, apparently, a skid plate.
I saw that there were 2 tanks, a 13 gal and a 20 gal. From what I have read, they look identical. How can I make sure I don't get a 13 gal tank? Are there identifying marks or numbers that would enable me to tell them apart?

The only difference in the tanks is how the vent tube is routed in the tank and you can change that.

Well then...no sweat. I'm going to do that little mod that gives you a 25 gal cap. anyway. Thanks.