Not a chance. To add what jcr said, my local library had really old editions of Etude magazine in the gift shop so I have read a few of those articles. However, you CAN find anything online:
http://etudemagazine.com/etude/1901/07/an-interesting-violin.html
http://etudemagazine.com/etude/1902/10/stradsvintage-of-1716.html
http://etudemagazine.com/etude/1910/08/the-london-violin-market.html (also has results of a violin auction--you need to adjust prices for inflation and the writer also mentions that some of the violins may not have been genuine or in very good condition.
http://etudemagazine.com/etude/1900/03/old-violin.html
Besides, even in your parents' or grandparents' time, a genuine Guareri violin would be worth a fortune. Unless one of your relatives was a successful (and wealthy) professional violinist, it is highly unlikely they would have owned such an instrument. And if they did, it would have never been thrown up in the attic and left to whatever atmospheric conditions exist in people's dusty attics.
Here are articles on how to spot fake labels (but violins that are deliberately made to deceive may have more convincing details):
http://www.skinnerinc.com/news/blog/violin-appraiser-how-to-identify-violin-label/
http://myviolin.co.uk/violin-labels.html