The best way to determine a frame's quality is to check its alignment, and then have the frame stress-analyzed by photronics, strain gauges, or finite element analysis. Next, test each joint to make sure the tube fails before the joint, and after that, cut the joints in half and look at the miters. Finally, send the ruined frame out for independent metallurgical analysis to determine the size, location, and severity of the molecular changes in the metal resulting from heat. If by then you're comfortable with the quality, ask the builder to build another exactly like it-don't change a thing-and this one you'll pay for.
Way number two is to look into the bottom bracket shell to see if the tubes are mitered. But what then-do you assume they're getting? Do you know if any of the tubes were reversed? It's not unheard of. What about material preparation? A dirty tube can't be brazed well, but you won't find out right away. In any case, meticulous pre-paint frame preparation, and a top paint job done in your current favorite color, can mask a lot. An unpainted frame can reveal a lot to an experienced builder; a painted frame reveals almost nothing to a bicycle shopper.
Don't get paranoid, just realize that the details that most determine a bike's quality are invisible. So, at some point, any purchase is a leap of faith. Sometimes bikes come with Lifetime Guarantees, but ours don't, which may seem odd given that they cost a lot and we go to great lengths explaining why they're worth it. But invariably, Lifetime Guarantees are:
1) Often abused. People figure they can wreck something and get it replaced for free;
2) A sign that the bike has little else to offer, was so cheap to make in the first place, and has so much profit built into it, that the maker figures, "Sure, why not, what can it hurt?"
3) Overall, fishy.
We do, though, guarantee that we put every effort into making your Rivendell the best bike you've ever seen, touched, heard about, or ridden. We use the best materials, the best craftsmanship, the best design. Barring accidents and assuming you're riding it for its intended purpose, we expect it will be the last bicycle you ever have to buy.