If it's a button like most other Fender basses I'm sure it will be like all the rest of their metal parts, available in either chrome or nickel plating.
The way to tell the difference is to clean it and then look at the metal where light is reflecting off it. If the metal has a bluish cast to it, it's chrome plated, if that cast is yellowish, it's nickel plated.
"Strictly speaking, there is no certainty, there are only people who are certain." -anon-