Generally the two most common mounts are Schneider(25mm) or Leica(39mm). Unless you were looking at odd lens.
I believe that most recent lenses are 39mm size. The 25mm were more common 30 years ago but you still see them on occasion.
Just check the diameter of the hole in the lens board. 25mm is about the size of a quarter so if it's larger it should be 39mm.