I have a Windows 7 product key that I foolishly did not label. Rather than downloading and creating disks for however many different ISOs exist (Basic, Pro, Ultimate, 32-bit/64-bit, upgrade, full version) and testing each one until one works, is there a way I can discover what the key is for specifically?
Use them at your own risk, though. Since at least PIDX Check seems to have originally been written for use with activation cracks, my first concern would be that it might try to "leak" the product key to some third party, so you may want to run it in some sort of sandbox and make sure it cannot access the internet.
I also found an open-source product key checker; if you're so inclined, you can review the source code to ensure it doesn't do anything unexpected with the product key, and then compile it to an executable yourself.