My grandfather and I found a violin in his attic that once was owned by a great uncle of mine who was a professional violinist. It appears to be very old and has a lot of neck wear where it was played for years. On the inside of it is a hand written tag that says "Jacobus Stainer....anno 1652", the middle part I can't read, but it is in another language, maybe italian or latin.

Now, I know that there were a lot of fakes made in germany in the 1800's of these Stainer's. I also read that one of the key things to it being a real Stainer is a handwritten ticket, which mine has, because it is hard to forge a signature. It appears to be written in like old fountain pen ink. I have pictures of the instrument which I can upload and link later. I looked at some of his actual violins, and the one I have looks like it is made in the exact same style, albeit in quite rough shape.

Can anyone help me out here? Are there any key things I can look for?

Take it to a violin shop or 2 (if you can) they will give better advice that we can here.

http://rperras.tripod.com/id20.htm

Here is a little of his history, for fun.