1709 Stradiuarius

Sadly you are in a field that needs 'hands on' examination by a registered expert.

Violins have been deliberately copied and faked for centuries, even while the original manufacturer was in business. So you can't even tell if it's a fake by date!

The label has to be carefully examined for known variations of the fakes, the woods and glues have to be examined, the varnish, and hardware likewise has to be compared to known styles. So on and so forth.
It's well beyond what we can do on this site, as you simply need hands on and very specialized expertise.

Real ones are worth Hundreds of thousands, fakes can be worth between $50 and a few thousand (depending on when where and what quality they are).

It will cost you to go to an expert, so you will need to decide how important it is to you. By far, most (pretty much all) 'found' ones are fakes but, like the lottery, sometimes...

Don't forget to also look at the Bow if it came with one. Sometimes the Bow is original and very expensive when the violin is not! They can look very unassuming!