All box cameras have a very simple shutter that's everything but accurate and can differ a bit from camera to camera of the same build. They usually have something between 1/30 and 1/50.
If you have another camera at hand, you could compare the sound. You may not be able do tell 1/500 from 1/400, but there the difference between 1/30 and 1/60 is very clear or at least clear enough for a good guess.
Don't know about the Certo Box, but for most box cameras, it's a safe bet to use 100ISO film for bright sunlight, 200 for cloudy and 400 at dawn or indoors. With the great exposure tolerance of modern color negative films, it works pretty good.