Authentic Stradivarius violins have "Antonius Stradivarius Cremonensis Faciebat Anno" inscribed on the label. This label indicates the maker, Antonio Stradivari; the town where the violin was made, Cremona, Italy; and the Latin for "made in the year," finished with a printed or handwritten date. If the label contains the words "made in" followed by a country, the violin is not an authentic Stradivarius.
Antonio Stradivari died in 1737, so any violin made after that time is not an authentic Stradivarius. The United States adapted its import regulations in 1891 to require the country of origin to be printed on all imported goods. Because this happened after Stradivari's lifetime, any violin with a Stradivarius label that indicates country of origin is a copy. There are only 650 known authentic Stradivarius violins in existence, and most claiming to be originals are in fact copies.
A more precise way to authenticate a Stradivarius violin is to examine the wood. An experienced dendrochronologist, or person who studies the age of trees, determines the age of the instrument from the tree rings in the wood grain. He compares the width and pattern of the rings to the wood used in previously authenticated Stardivarius violins to determine if the instruments were made from the same tree.