This image is from http://www.cemetarian.com/. The website has a page where you can click on a company name to go to a history and dating site. Pattern companies repeat numbers when they get to a certain point, so you need to know the approximate age of the pattern. Ballpark, using indicators (hairstyle, silhouette of garment, etc.) This is a really fun time sucker. I have found it useful because the copyright date is not always on the pattern.
Click on the tab that shows the change in standard sizing through the years. This helps explain why a vintage pattern size 12 is not the same modern size 12 we buy today.
Cemetarian also has an ebay store. I have never bought from them but they do have an excellent ebay rating.