If you can't see any telltale flow marks and the ceiling stain is quite small, look for “sparkles” at the bottom of the ceiling. A polish is a nail that did not reach the structural element. Moisture that escapes to the cold attic from the rooms below often condenses on cold nails. Sometimes you can detect this if you go up to your attic on a cold night.
The nails will look white because they are frosted. When the attic warms up a little during the day, the frost melts and drips, then the nails freeze again at night and so on. The solution is simply to cut the nail with a side cutting pliers. A striking aspect of homes in the United States is the flimsy quality of even the most expensive ones.
The houses are literally built like a house of cards. Weak beams, plywood, flimsy insulation, flimsy siding, and roofs that come off in high winds or simply rot after a few years. It's no wonder that in tornado or hurricane season, houses are literally uprooted from their foundations and thrown into the air. I can't imagine having a mortgage that I have to pay for the rest of my life for the house that will collapse before I die, but that's typical in the United States, I had a friend who said he has to repair the wall because the insulation fell and is freezing, imagine that happens in a house in the United Kingdom.
I don't know where you are in the United States, but no, 50-year-old houses aren't just torn down to make room for new ones. A wooden house with properly sized beams, minimal insulation, and a cement base is reasonable enough for the environment, but you don't want to pay a million dollars for it unless you really like this climate. It's been through nearly 30 tornado seasons, a tree has landed on it, it's been through record winters and summers and prepares for storms every year, but my house has lived to tell it lol. A family member of mine recently bought a new house in Los Angeles for close to a million dollars, and it's also a plywood construction.
The first house was on a golf course (important for centers) with a fishing pond nearby (important for my son). The other point I would like to make is that you are right about the big developments that occurred in the 1950s, and all of them are pretty standard houses. American houses sometimes have the appearance of brick walls, however, these are simply glued to the outside of the plywood walls, giving a false sense of quality and strength. Your house is big, wow, it's still likely to be a piece of garbage that looks like any other house in your neighborhood.
And my house in Sweden (which all my friends and colleagues said was very good compared to American build quality) is built very cheaply. There is a neighborhood near my house where the houses were built in the 60s, I think, and a lot of aluminum wiring was used. In the United States, a 50-year-old house is considered old and demolished to make room for another flimsy but expensive structure.