Use a flash meter if possible. It works like a light meter, but measures the flash burst and tells you the aperture/exposure to use at your chosen ISO and sync speed.

If you don’t have one, a practical workaround is to use a digital camera with full manual control as a test tool. Set the digital camera to the same ISO as your film, put both the camera and flash on manual, and adjust aperture/shutter speed within your film camera’s flash-sync limits until the test image looks right. Then transfer those settings to the film camera.

Historically, photographers also used Polaroid test shots to verify flash exposure and lighting, so the digital-camera method is the modern equivalent.

A few notes:

• Keep shutter speed at or below the film camera’s flash sync speed.
• Flash exposure is usually adjusted mainly with aperture, flash power, and flash-to-subject distance.
• Take notes so you can compare your digital tests with the developed film and refine your setup.