Prime magnitude and keys...a ?

Hi everybody, I was wondering if anyone is aware of a function or test which would allow a person to feed PGP or other RSA algorithm a test key and then look at the result and determine if the key was greater or lesser than the actual key? I am looking through several books and so far have come up with nada. I was hoping that somebody more familiar w/ the field would offer a suggestion. Thanks for you help ahead of time...