How do you know when an LP is a first or early pressing?


Being relatively new to analog, there has been a steep learning curve.
The latest important bit of knowledge has pointed me to the codes stamped in the runout groves, the labels and the sale listings. The questions are how to read the codes, and what to look for on the label and on the sale listings? For instance, a friend guided to to the “pink label” British Island Records pressings. Believe these are all first pressings and the original British. I bought a couple including Emerson, Lake and Palmer’s first album. A pretty good used one cost me $25 which I understand is a good price. I would like to find and buy more of these early, or first, pressings. I understand their sound quality is normally, or frequently, excellent.
mglik
Discogs is your friend
https://www.discogs.com/

It's not 100% accurate for listing EVERY pressing, but still the best resource,so far.
SQ varies with every copy.

You can save some time here, but it you will pay for it.
https://better-records.com/

mglik, it can go either way. Although the masters may be better back in time popular records often got cut short in pressing quality. A good master with top notch modern pressing technique and premium vinyl can yield a superior result. This does not take into account that most popular records were not taken the best care of. I personally will not buy used vinyl. I have thousands of old records. I do not need any more. Some reprints are actually far superior to their original versions because the original masters were terrible. A good example of this is the early Roxy Music records. The 1/2 speed remastered versions are worlds better than the originals. Some of Frank Zappa's early catalog were mastered on second rate systems. Before he passed he remastered several of them with exceptional results. Far superior to the originals. 
The site is down. If they're left coast the inventory could be melted.
Awww
...
+1 Discogs. Great resource.

I’ve cataloged all my physical media on Discogs. Takes much time (depending on how many you have) but worth it.

But also agree that not all first pressing are the best, even though they might be worth more in some cases. Many times a reissue/remaster can sound better. It just depends.