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

Discogs usually has all the info you need. The real fun starts when your buying records that Discogs doesn’t know about (yet)!

Then you can upload all the info yourself.
If there is not info yet then it will be there in the future, even if the record is extremely rare. The progression of discogs is huge, I remember when it was useless source, now it’s the best place to find all info about records, everyone can add or change info (if there is an error). 

More deadwax info about first pressings
https://www.analogplanet.com/content/proof-1a-1b-and-1c-columbia-records-are-all-first-pressings

Some speak so highly of original pressing SQ. As others have stated this is a quite variable topic that cannot be black and white. What can be is each pressing’s value. If this is important to you then the original pressings often have the highest value and you hope better SQ as well. There are often reissues that have excellent or better SQ, like MoFi and QRP. It takes research and simply buying multiple versions and let your ears decide.

I have not yet taken the leap for a hot stamper, but with the 30-day guarantee, I will likely do this soon. It really makes me wonder how many I have in my collection. My original copy of Bruce Tunnel of Love sounds amazing so I like to think of it as a Hot Stamper : )

You must not be on Discogs so that is definitely your first step. This is not only to know all versions released but to catalog your collection.

There are two distinct inquiries here- one is how to determine if a record is truly a first pressing, which not only involves an examination of deadwax nomenclature, but label, including logo, sometimes the packaging, and what country of issue as well. There’s a lot of minutae in this but the info is out there on various websites for different labels, bands and genres and Discogs is a good place to start.
The second area of inquiry is connecting this information with sound quality and that’s more difficult in some ways because it depends on listening evaluations and comparisons. You can do this yourself if you have the time, money and ability to source multiple issues. You can rely on anecdotal comments from others who have done the comparisons. For classic rock, the older threads on the Hoffman board are good, at least those that are in depth and not simply comments stating that their copy "sounds good." The Better Records/Tom Port thing is a whole other subject that is fairly controversial and I’m not going to wade into that.
In some cases, with rare jazz records, there aren’t numerous pressings- often just an original run and sometimes a second pressing, followed years later by reissues, often with a big question mark about source- needle drop, digitized, etc.
I have found, in the classic rock area, that the sonic differences do not necessarily correspond to any rational relationship with when the record was issued-- oddball reissues by the label itself that sound better than first pressings, differences in EQ and tonal balance that may favor a later (but still early) issue over a later one. There are also "known" hot pressings of some of these records. I think it is very hard to generalize and the knowledge that has developed over the years, after the fact, by collectors and sound hounds (two different species) is often siloed by band, particular album, or label. There are also shortcuts in some instances that can save you money where the metal parts from an early desirable pressing show up on a later, much cheaper period reissue. The quality of the vinyl used at the time is also a factor, so there are many variables. I have found that I have accumulated many copies of records I like but aren’t the best recordings or pressings and the end result is comparing different sets of compromises, occasionally resulting in no one "best" sounding copy, but shadings of difference with various strengths and weaknesses.
This is, as EBM said, something that you develop over the years, with accumulated knowledge based on first hand experience having gone through a lot of records. I have done purges over the years to get rid of thousands of records that didn’t make the cut and got sidelined-- to the point where I needed to make space and saw little need to keep the lesser copies. That said, I still have some albums where I probably have a dozen different copies. This was not the result of just a few years, but decades of buying.