I'll have to reply with what some Supreme Court justice said about pornography - "I know it when I see it." Really, look at nickel plated guns and at chrome plated guns and you will see the difference. In words, nickel tends to have a yellowish or "warmer" look, while chrome has a bluish or "colder" look. At one time, around the 1890-1900 period, nickel plating was so common in the gun industry that the majority of some S&W models were nickel plated, and some other companies made whole lines only in nickel. Nickel plating was new at the time and made things look "high class". This was true of not only guns but hundreds of other items (Google "nickel plate line" for one interesting story).
Later, nickel plating was common on guns to be used around salt water or carried in a shoulder holster where perspiration would ruin the finish on a blued gun. Today, stainless steel has taken the place of plating for protection of the finish, and nickel plating has gone out of style, though S&W and some other makers will still do it on special order.
Chrome plating is fairly recent, and was uncommon on guns until the advent of cheap handguns when it was used to give a flashy look, appealing to some people. It is cheaper than nickel plating and easier to do.
Jim