I think that Tom Port of Better Records has done a wonderful job identifying differences between vintage recordings and current reissues. He writes wonderful descriptions around specific titles around "hot stampers"--copies of exceptional sound. These are often pricey, but it a good online resource. He has detailed information from various weekly shoot-outs and while opinionated, seems to find the diamonds from the coal. He is also quick to point out shortfalls around a title, especially around the newer vinyl. Read up on "Aqualung" and Led Zep's "Houses"--classic "butcher" jobs.
I always look for a trusted vinyl shop where you can pre-listen to the vinyl. In LA, where I live, I especially like Rhino Records in Claremont and Equator Books in Venice Beach. I have found a lot of great vinyl digging through their bins.
For the maximum number of albums, I also like Ameoba Records in Hollywood and SF. You can usually meet up with seasoned collectors who will freely offer their opinions up. I have been steered to a lot of great artists this way.
I also think that magazines like Q and Goldmine are wonderful endorsements of back catalogue.
