In his 1998 book, Yellow Sky: an alternative perspective about the human condition and history, Kenneth Lipton argues that high radon levels in southeastern Pennsylvania may provide evidence of prior high tech civilizations.
Isotopes of 7 elements could be found in small amounts 26,000 years after use of nuclear weapons, so it would be pretty easy to figure out if they were used in pre-historic Pennsylvania. Furthermore, radon is produced by decaying uranium or thorium, which just means that it's a good place to mine for large amounts of uranium (such as eastern Pennsylvania) and doesn't in any way indicate that there are small amounts of uranium waste there from weapons use.
Material Produced By Nuclear Weapons: All nuclear weapons derive at least part of their energy from nuclear fission. Without getting too technical, this essentially means you whack an element with a large nucleus, like uranium, hard enough for it to split into other elements. These elements are called fission products, and most are too unstable to stick around for more than a couple hundred years. So, they would likely not be detectable 26,000 years later.
There are 7 long-lived fission products that have half-lives between 200,000 and several million years and would definitely be detectable if produced 26,000 years ago. These elements can occur naturally, as natural nuclear reactors are possible, although they are very rare.
If these fission products were found in any abnormal amount outside of such a natural reactor they would indicate that nuclear weapons were used in the area. I can't find any information regarding large amounts of them being found in Pennsylvania, so there doesn't seem to be any evidence of nuclear weapon use as the books suggests.