Where's All the Placer Gold?
You will need to be sure you are not trespassing when you decide to go out and do some panning. Existing claim owners can be aggresive in defending their claims, and the laws are on their side.
Placer gold is gold that has washed from its original location by natural wind and water erosion, like rain, and can be found on hillsides and in riverbeds. The larger nuggets are closer to where they have washed down from and smaller bits and flakes get washed further and further from their source. Even tiny gold flakes have value, but we all want to find that nugget that leads to the source.
There are several books for different gold rich areas, all with maps and directions. With your own due diligence you can still find good placer gold even on public land and these books give you the details and information to get going.
Amazon has informational reading for all areas where you might want to get started in finding gold, they also offer books on rock and mineral identification and all the tools and supplies for panning and prospecting. Mineral identification kits can help you be certain right on the spot so you don't waste time working a site for the wrong thing.