Most of the places you might find a nugget are claimed. There are occasional big nuggets in Colorado, and many people find these in areas where dredges ripped up the bedrock and left huge piles of river rock where they worked. The area between Breckenridge and Fairplay is a good example (there are a few places you can detect or pan/sluice/highbank/dredge in these areas, but not very many, unless you know the claim owners). There are a few Colorado places that you can work where its open to anyone (Point Bar along highway 50 or Cache Creek near Granite), but these are not known for nuggets (and you also need to consider not working where the water is now, but rather where it might have been 10,000 years ago).
There is a campground in Fairplay that is on top of a tailings dump - if you stay there, you can pan for free (but you will need an RV - they have power, water and sewer hook ups, but no facilities for tents or on site rest-rooms/showers). I was there 2 weeks ago to scout it out for a summer weekend outing (Its right at the bridge on 285 at the east end of Fairplay) - Looks promising, but expect to move a lot of river rock to find anything if you stay there. This RV area is very close to where I found my larger nuggets with a White's GMT metal detector.
Any place in Colorado's mineral belt that you see a waterway and huge piles of river rock, its likely been dredged, and also likely to contain gold that was missed.
Cripple Creek is known for Gold in Calverite (arsenic/pyrite) - your metal detector won't find it. You have to know what it looks like - a lot of miners threw out the best ore there, because it did not look like gold to them. Its being reclaimed now.
In Breckenridge you will have a lot of trouble with the law looking for anything within town - they prefer you to shop there instead.
