How do i prove that a quit claim deed was forged?

File a lawsuit in county court (or possibly federal court, if the property is in another state and worth more than $50k) seeking a "declaratory judgment," or court determination, that the deeds are forged or false. In  the suit, seek also a "injunctive relief," or a court order that the property cannot be sold pending determination of the lawsuit--you actually apply initially for "temporary restraints" (e.g. a "temporary restraining order") pending an initial hearing on whether an injunction should issue, barring sale until the matter is fully resolved. You need to get injunctive relief to stop any sales from occuring while you are trying to prove the deeds were forged.
The evidence you can sue to show that the deeds are forged include: the will, which shows your uncle's intentions for the property; testimony of people who knew your uncle and can testify to the relationship, or lack thereof, with this person (i.e. why would he do this?); handwriting analysis (to show that your uncle's signature is false). The other side in turn can try to show that there is a good reason for what your uncle did--for example, that your uncle owed this person something and that person forgave the debt in exchange for the deeds.
You will need an attorney to do this--procedurally, this is a complex case, with all the orders, etc. you need to apply for. You will also likely need a handwriting expert. Unless the property is worth at least several tens of thousands of dollars, it is not likely worth the time and cost.