How to determine your BB threading:
1) Measure the width of the BB shell itself, excluding lockrings and cup flanges. Italian BBs are 70mm wide, whereas everyone else's are 68.
2) Try to thread a known English/ISO cup (marked 1.37x24) into either side of the shell. If it slips in and out of the shell without engaging the threads, you have an Italian BB.
3) Follow the threads on the right side of the BB shell. If clockwise motion pulls your finger or nail inward, your BB is French or Italian. If clockwise motion pushes your finger or nail outward, your BB is English or Swiss.
4) If the BB cups are stamped 1x35, you have French or Swiss threading. (Adjustable cups of the two systems are identical.)
5) The toughest challenge is to distinguish Swiss from English, since the diameters are nearly equal and the threads differ by only a few percent (24 TPI vs. 25.4 TPI). English cups will thread a few turns into Swiss BB shells, and can be forced further.
6) The other challenge is to determine, on a late 1970s French bike, whether it is French or Swiss threaded, so that you know which way to unscrew the fixed cup.

Italian: big diameter and wide shell
French: standard diameter and shell, RH-threaded fixed cup