You can compare the readings of the film camera to those of another camera of known accuracy. Both cameras must be set to the same shutter speed and aperture and aimed at the same target in the same light.
Itís usually the case that no two meters read precisely the same, but the difference should be small, usually no more that 0.2 stops apart.
I have a number of old cameras that generally agree with each other within this tolerance. One thing that photographers have always done is to adjust the film speed based on tests.
For example, you could shoot several different frames of 100 speed slide film at ASA64, 80, 100, 125, 160.
Each of these are 1/3 stop apart. Choose the slides that look best and use that as the film speed setting whenever you use that speed film. This is called finding you best EI (exposure index) that gives the most correct looking slides.
You can shoot a friend holding large sheets of paper with bold numbers: 64, 80, 100, 125, 160 corresponding to the film speed setting of your meter and choose the EI for that film speed based on the resulting slides.