In the old days (before the web) you could not read the operating manual or instructions for an appliance, device, or tool until you got it home and unpacked it. Getting the manual was considered one of the benefits of purchasing the product. In fact, you had to purchase extra copies if you lost the original, or wanted to check it out. It was often only later when you finally had the box opened that you discovered a) it did not permit the function you bought it for, or b) it was a quarter inch smaller than it looked and so didn't fit, or c) it was incompatible with the assessors set you already had, or d) it had no manual!
Those days are gone. You can find a PDF version of the manual for most products on the web if you search hard enough. It is not as easy as it should be, but the smarter manufacturers make it easy to download the specs of whatever they sell.
That leads to this new rule: get the manual first, before you buy.
For a large home remodel I had to purchase a pile of new appliances, lights, plumbing fixtures, hardware, materials, gadgets, and some tools. I instituted a "Manual First, Buy Later" policy, and it had immediate positive effects. Once I identified a possible candidate for purchase, I would google for its manual. Equally important as finding the operating instructions and basic specs, is to get hold of the installation instructions. There are few sites that aggregate manuals and specs of major lines, but often I would wind up at the manufacturer's site. There I would download the PDF and read it carefully. That's where you find out its precise dimensions, its actual power needs, its exact connections, its real compatibility. I lost count of the number of inappropriate bad purchases I avoided by studying the manual and specs first.
What baffles me are the clueless manufacturers who still don't put their installation and operating manuals online in 2012. (I'm thinking of you, LG.) The main result of this process is simply fewer surprises. Less returns, better integration.
I was heartened to see that even the professionals do this. Here is a snapshot of our plumber "at work" in the bathroom. He has his tablet opened to a installation PDF, and his phone is googling a help number for questions brought up by specs in the PDF.
Locating any particular item's installation and operating specs is still not as easy as it should be. Amazon could make it the norm to have the full spec PDF for every item they sell, or Google could try to algorithmically sort them out, or some clever aggregator could centralize them all. But for now it is worth seeking them out first, any purchase later.