It should have a sticker with the date of mfg. on the right side radiator guard.
You can also just LOOK at the tank and the fuel pump/sending unit connection. The difference is VERY obvious.
Built 8/20/07 and before:
Built 8/21/07 and after:
There's a bunch of them on ebay once you determine which you need.
However, many times you can just pull the unit out of the tank and it's simply that the clip that holds the float comes loose something easily fixed.