You're asking some good questions - some of the right ones to ask, in my view, since they indicate a desire to judge by the product of the work and not necessarily the name on the decals.
A couple of points:
1. Yes, brazing takes skill and care to do well, and this of course to some degree determines the integrity of the frame. But there are other aspects of the frame that take as much skill, and certainly more time - design, development and execution of little "signature" details, paint and graphics choices, selection of frame parts, including tubing, to meet the needs of a particular rider and intended use, and working the lugs into shape.
2. You can't always tell when a frame was carefully built, at least without the paint off. But there are often good clues.
3. Some frames from master craftsmen were constructed with shortcuts.
4. Beginning in the early 80's in Italy, factory production methods and economies of scale did indeed enter the Italian frame building industry in a big way and change the way many racing bikes were built.
5. The idea of the one-man shop was always a bit of a myth. They existed, but most good framebuilders would find additional hands as soon as they were big enough and could afford it. In some ways, though, supervising a small production shop and teaching and maintaining the necessary level of skill, and assuring the quality of the product, takes just as much passion and commitment as building every frame personally one at a time.

The biggest change to the meaning of "hand made" in the steel era occurred in the early 80's, when investment cast lugs swept through the bike building industry in Italy in a matter of a few years. This allowed for greater precision in the building of frames, and in some cases increased structural integrity. But it also reduced the amount of time it took to build a frame, and made many of the economies of scale and larger-scale production methods more feasible. Prior to that point, most bikes used pressed lugs, which required considerable hand work to make them aesthetically attractive, and also allowed for greater manipulation by the builder to make them distinctive (i.e. the way the lugs were thinned and profiled, and the way those chosen lug profiles introduced a certain style of tube transition, giving the frame an overall "look"). Investment cast lugs generally received very little treatment before being brazed up, because they were considered aesthetically acceptable right out of the box, and this saved labor, hence time, therefore money. This is when you tend to see uniform lug thickness and "plug-and-play" construction - where frames wind up looking a little bit like tinker toys, with distracting elements like cast-in logos everywhere.

It's certainly true that a lot of earlier Italian frames were a bit crude in terms of finish and sometimes embody less than perfect brazing. The idea that each frame built by an Italian master was made with elaborate care to a standard of perfection is part of the myth. In fact, there was an emphasis on making frames to be raced rather than displayed, and a tendency to look down on frames with too much finish work as "filed to death." This attitude may have changed a bit when some US builders took finish work to a new level and American customers' expectations changed accordingly.

But having said that, even many earlier frames were indeed built with considerable skill and care to a high standard with a great deal of pride and investment of time. It's just a good idea to maintain some perspective when looking at individual frames, rather than carrying the level of expectation one would bring to the viewing of, say, a Renaissance masterwork. A certain amount of appreciating them has to do with learning to recognize the little "signature" flourishes and choices mentioned earlier. This may help offset the disappointment of finding file marks.