The first thing you should do is look up the mining laws for your state as they vary from state to state. While California doesn't require claims to be clearly marked, Arizona requires it. As I've never prospected in Colorado I have no idea as to what they require. I feel that this is something that the feds should require of all claims filed on federal lands as it would get everyone on the same page for a change.
The LR2000 info is not always up to date so that can be a shot in the dark much of the time. Your best bet is the county recorders office so you can see if there has ever been a claim filed for the area you want to check out. If there's never been a filing with the county, then is a safe bet that it's a clear area as this is always the first step in claiming a mine. If you find that there has been a claim filed for that area, then go to the LR2000 and see if it's current.
There is a new reqource that will be coming online very soon that will make checking claims and their status much easier for all of us. It will cover all states that have mining claims in them and you'll be able to determine the status very quickly. I've been helping to beta test the new site but have been asked not to give out the address for it until testing is finished up and they're ready to go public. They're hoping ot go public with it in the next week or so. This site will include maps that will show claim locations as well as geological maps for pretty much the entire western United States. WHile on the claims map for say Arizona, you will be able to click on a map section and it will list all claims within that block as well as provide links directly to the LR2000 so you can check the current status of any claim. There will also be tons of other information on this site and it will be growing very quickly. Articles, laws, map of all types will be available for all outdoorsmen. Although this is going to be a very complex site, it is being setup with the end users in mind. It's very easy to navigate and very pleasing to the eyes. (No excess ads etc.) It's going to make doing research a LOT easier for all of us that are serious about mining!!!
That you're attempting to do your due dilligence speaks well for you G.S. It's an unfortunate fact that there are many that don't even bother to see if they're on someone elses claim before they go out digging. Now days with the internet resources available to everyone for free, there's no excuse for not doing your due dilligence. To make it easier for everyone until the new site is open to the public here's a link to the USGS maps page. Files are zipped PDFs so you'll need both an unzip utility as well as Acrobat Reader on your computer. All the maps I've gotten from them so far have included the aerial view as a layer that can be turned on and off.
The USGS Store - One stop shop for all your maps, world, United States, state, wall decor, historic, planetary, topographic, trail, hiking, foreign, satellite, digital
When you get to the site, zoom in to the area you want to get the map(s) for. As you zoom in closer, all the names for the maps will come into view. Then click on the "Marker" button on the right side and then click on the map you want. Now, click on the marker and this will bring up a list of all the maps versions available. Many have versions going back to the 1800s as well as several scales for the images. Then choose the version you want by clicking on the download link. There is no charge for downloaded maps but if you request a printed copy they will charge you. As they are PDF files, you can print them out yourself or take the files to Office Depot or some other office supply that does printing and have them do them full size for you. As I haven't had any of them printed yet, I can't say how well they turn out. All of the ones I've gotten have been at 1:24000 scale with 40ft elevation lines.