http://www.philatelicfoundation.org/Please note that the difference between a common and rare variety of the same stamp is a VERY slight difference in paper, watermark, perforation, or color difference. If you look through your catalog, you'll see that there is often a common variety worth a few dollars and a rare variety worth many times more. In most cases, there are many millions of the common variety (hence, why it's common) and a few of the rare variety (hence, why it's rare).
If you post a good-quality scan of the things you think are rare to this or another message board, people here can help you to make the initial determination of whether there is even a chance that it is the rare variety.