The search function in the Bike Forums C&V Subforum is by far the best resource I know on this topic. A lot of time spent on the internet, Craigslist, Ebay, etc combined with the reading of appraisals and remarks by forum members is really all you need. But, there are some key features that might help you narrow down your potential buys. For me, some big hints as to bicycle quality (or at least their position in the hierarchy for that year) are the dropouts (stamped vs lugged) and the tubing (Reynolds 501/531/753, Tange Infinity/#2/#1/Prestige, Columbus Cromor/Tenax/SP/SL/SLX. I'm not as familiar with Ishiwata, Fuji Valite, or Vitus. Miyata had their own tubing. Another hint as to quality is the groupset. Information on groupset hierarchies can be found on this forum on via Google. Lastly, pick a few favorite bicycling marques and learn about them. Read some vintage cycling catalogs. Then you can get an idea of where a model might fit in the hierarchy and what you premium you should expect to pay for it.