How can i find out how much an item signed by Patrick Henry is worth?

Probably a great deal of money. The best solution would be to find an antique dealer or auctionneer that would know the approximate amount you could expect to receive for the item at auction. This will give you a good idea of how much the item would be worth both for your own records and for insurance purposes.

Additionally, if you watch Antiques Roadshow on PBS you may be able to see a show where they have other founding fathers signatures on similar items. That would also be able to give you a ballpark figure for your item. Sorry I don't have any actual amount for you.

These might point you in the right direction.

Baurman Rare Books:

http://www.baumanrarebooks.com/

http://www.baumanrarebooks.com/contactus/contactus...

Find an Appraisal Expert

http://appraisers.org/findappraiser/index.cfm?CFID...