Five questions, one verdict. The diagnostic that decides whether the panel in front of you is pre-1940 cylinder glass — and the conservation-grade protocol it actually wants — before a steel scraper does damage that cannot be undone.
Methodology cites Wade Marler's historic-glass piece (IWCA pre-1945 reference; NPS Preservation Brief 33; Bendheim restoration glass technical).
The cylinder-glass process that produced pre-1940 residential window glass was replaced by the float-glass process over the period roughly 1959 to 1975. Pilkington patented the float process in 1959; commercial US adoption was substantially complete by the mid-1970s. Properties built after about 1975 are essentially never going to have cylinder glass — even if a homeowner has installed reproduction or restoration glass, the original panels of a post-1975 building were float glass from day one. Building age is the first and largest filter for whether you are dealing with heritage glass at all.