You can tell quickly by test-threading an ISO adjustable cup into the left side of the BB.
With the usual caveat, my 1980 Peugeot PKN-10 was Swiss-threaded. I force-threaded an aluminum Sugino ISO fixed cup when I switched to a Sugino triple, and it has held nicely for more than a decade.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069