Cast lugs are a lot better for silver brazing, because of close tolerances. They are much easier to finish, stamped lugs were a pain to clean up because of the welds. Stamped lugs are usually somewhat misshapen. Other than a pleasing shape and generally smooth finish, the aspect of lugs that builders really go after is called the "shore line." That's the transition from tube to lug. It's the hardest part to get nice looking, because you don't want to file into the tube.

You would hope that the miters touch perfectly, that's a major component in frame strength. I have seen a high-end frame where they didn't. That was depressing. I don't know what you can tell from looking in the bottom bracket shell. I usually miter the seat tube to the down tube. On a cast bottom bracket, there is a clean line to scribe a miter to the bb diameter. On a pressed steel bb, there is no such clean line. Very difficult to judge the internal joins.

Usually, when people in here praise the lugwork on a bike, I have to bite my tongue. Modern lugwork has gotten a lot cleaner than most of the stuff done in the classic period. Those bikes were made to look good from standoff distance, not fingernail distance. And it really doesn't matter from a perspective of the bike's ride.