The plastic tanks do fade from the sun, but usually just look bad.
Is this one with the vent screw in the filler cap? Just put a little gas in it and close the vent, then let it sit in the sun for a short time. If it starts to balloon and you you don't see any gas leaking as you slosh it around, I'd say you have a good tank. Then loosen the vent to relieve the pressure.