Any advice on buying quality vinyl

Uberwaltz, I reached the same conclusion doing a test of three recordings of Madman Across The Water.  I played Tiny Dancer from all three records and the one that sounded the best , save for pops and clicks was my original Uni pressing from way back.  How are you able to determine what is an original pressing?

I think it is impossible to generalize. The attraction of the better audiophile reissue labels is new, usually clean playing LPs that may or may not sound better than an earlier pressing. It depends on the particular record, which usually means you have to dig down and learn about different pressings, compare them or rely on anecdotal comments from other people who have made such comparisons. I don't think there is one easy path.
The biggest limitation can often be the selection of material offered by the audiophile approved labels- in many instances it is the same stuff, reissued repeatedly, because there is an established market for known "best sellers." That said, if you need to fill out your collection of certain rock, jazz, etc. they are worth considering. 
But, if you want to push the envelope and look for less well known records, or are seeking out some records that have never really been bettered by modern masterings, you are in the world of older pressings. It is, for me, a continuing education, not just of the minutiae of pressing details (and how that correlates with sonics), but an ongoing exploration of different music that I wasn't familiar with, or simply ignored. This journey has led me to a lot of music I was unfamiliar with. 
For example, I had lost interest in straight ahead jazz. I had many of the warhorses- great records, to be sure, but I seldom listened to them. A friend turned me on to "spiritual jazz," a movement that took place in the early '70s after mainstream jazz became a dead letter commercially. I started to tap that vein- records from small labels like Strata East, Nimbus West and even smaller, private labels. Some have been reissued (Pure Pleasure has done a number of them, and despite their opaqueness about source material, the ones I've bought sound quite good and are far cheaper than the original pressings, condition is less of an issue, etc.). Speakers Corner did a remaster of Herbie Hancock's Crossings, another eclectic spiritual jazz record that is cheaper and in my comparison, sounds better than a time capsule original US pressing.
I think you have to figure out an entry point. Are there particular records you are seeking? You can research anecdotal comments on "best" pressings. Are you looking for new to you music? Lot's of things to read, and sample. There are any number of lists, including comments here and on other fora, about what people like.
This sounds like work, but it is fun work, and you learn quite a bit along the way about music history, pressings and your taste. I am also listening to a lot more different music than I did 20 or 30 years ago and really enjoy finding things that are a little off the mainstream path. Treat it as an adventure and you will be rewarded accordingly.

"I think it is impossible to generalize."   +1(a plethora of variables)   

+1 with whart on this. It’s work and you have to be patient and visit used record stores and thrift stores when possible and learn (very important) how to visually "hear" the quality of the groove as you tilt the record in good lighting to see and evaluate the condition. Having said that, there will be disappointments. I like stores that allow me to listen to a recording, at least a track per side of my choosing. Discogs is good. I don’t trust Hot Stamper stuff. MoFi is hit or miss. Classic Records has, for most part, been ok, but I got burned with a defective Peter Gabriel 2 on the lead in. Back to Black has consistently been a good bet.

Good luck! This is part of the fun because, when you get a really good pressing, it makes the effort all worthwhile!

And, if you don't already own one, get yourself a good record cleaning machine!

How are you able to determine what is an original pressing?

thats what Discogs is for, and also the vast majority of sellers there are reliable and of course you get full pricing data. Even if buying from other sources you should use it to check prices