Use Supreme Court Justice Potter Stewart's test for pornography: "I know it when I see it." Serious answer- It is by sight only, which is one of the reasons that many collectors, myself included, aren't collecting this "variety". One of the more experienced collectors told me that certain cleaning methods will change the rim color. One author of a Nevada strike price guide has a copper rim variety on ebay with .999 on the rim and had a buy it now price of $450. Seems like the casino used an old LM mint die to produce a no longer allowed .999 on the outside rim which is a new copper rim. The current bid price is surprisingly high for a strike that was in play for over three months.