i always check using the rewind crank to make sure my film's loaded after advancing the first frame on my film cameras.
one of the nice things about the m2 and m3 is that the spool that holds the film grips pretty tightly - you can be very certain that the film is winding correctly if you rewind and it won't turn anymore. never had a problem with the m3 but i have had trouble with cameras like the contax II/kiev 4 where the spool isn't very high quality. sometimes the film leader comes out of the hole in the spool.
need to be extra careful with those.
Epson R-D1, Ricoh GRD III, Rollei TLR's, Lots of others