If you go on
http://www.discogs.com/ you can find out all the release information about disc and vinyl albums. Let's take one of my all-time favorite albums for example, In Keeping Secrets Of Silent Earth: 3 by Coheed and Cambria.
It sorts by year ascending. So as we can see, EVR87 is the code for the first pressing of it (whether CD or vinyl), done by Equal Vision Records (their main label at the time). The ones that say B-Unique I'm not sure about (and weren't aware about), but the Columbia ones were because they were signed with Columbia as well. There was a vinyl pressing of black (400 pressed), orange/white (400 pressed), clear (300 pressed), and blue/grey (300 pressed) records at the time. They command CRAZY prices ($500+!!!) online, so earlier this year, they re-released it on vinyl, and 88843099221 is the re-release code (and as you can see, it's in my wantlist!) So basically, to figure out the first pressing, go on discogs, go to a main release page for an album, and go to the first release in your region, and that's the first pressing for all intents and purposes. Beyond that, we're talking test pressing and shellac territories, and that's far beyond me.