Examine the inscription that can be found on the inside of the violin. Original Stradivarius violins bear the Latin inscription "Antonius Stradivarius Cremonensis Faciebat Anno." The year that the violin was made will also be inscribed. It is important to remember that fake Stradivarius violins will also bear this inscription.
Contact an expert to have your violin appraised for authenticity. The best place to start is the American Federation of Violin and Bow Makers. There you can find an expert who can appraise your violin.
Look at the varnish. One of the most important ways an expert can tell if a Stradivarius is genuine or not is by examining the varnish. Stradivarius used an exquisitely translucent varnish that only he knew the recipe for. A true Stradivarius looks as though the wood has been sealed in a time capsule, and to this day no one has been able to figure out how to emulate the varnish.
Examine the wood. Throughout his career Stradivarius changed the woods he used for each violin he constructed, constantly trying to find the best combination for his instruments. All of the various combinations of woods and glues used have been meticulously catalogued by scholars. Only Stradivarius experts know of each combination and method, for each of the 1,100 violins made by Stradivarius throughout his lifetime. By utilizing this knowledge they are able to tell instantly if your violin is a copy or a true original.