Did you check the following article? You can register these keys on our site and there is also a useful tip for finding and identifying keys for older versions.
It is not clear enough what is the goal, but what I can suggest:
1. Parallels 3.0 license keys are 25 characters. Parallels 4.0 license keys are 30 characters.
2. You can always find out which key is used by Parallels by going to: Help -> Activate product. You will see your key there and a message that product is active.
3. If you see, for example, an upgrade key and wish to find what were the previous keys, you can check the following article: http://kb.parallels.com/en/4661, there is a license checker and a direct way to check the keys on your Mac.