My grandfathers violin has a label inside marked Stradivarius. How much is it worth?
Strads (and other “name” makers such as Amati and Guarneri) are so valuable and rare that the chances of finding one that is undocumented is virtually nonexistent. Many makers (particularly mass production “workshops”) used Stradivari or other labels to simply identify the model of the instrument, or, in unethical cases, to inflate the price of the instrument. This doesn’t necessarily mean that the instrument is worthless, though. A violin maker will be able to give you the best information about that, and Kevin is happy to recommend one for you.