Since you do not have the appropriate test equipment, the best test will be to shoot a few test rolls and examine the outcome. That being said: Seek out a local minilab photofinisher; they should have an instrument called a densitometer. This is a device that passes a beam of light through developed photographic film and assigns a numerical value to the amount of light that makes it through the material.

Procure a gray card from a photo dealer. The gray card has gray surface that reflects 18% of the ambient light. Use a film camera loaded with a moderate speed film. Compose a picture making sure the gray card is predominant in the scene. Make sure the image of the gray card comprises about ¼ of image area. Use the light meter to read the exposure settings using the just the gray card as the target. The meter placement must be such that it receives only the reflected light from the gray card.

Shoot a series of pictures that include and bracket the meter reading setting. I suggest 1 or perhaps ½ f-stop increments. Use placards included in the scene so you can correctly identify each frame and the exposure used. Use only the camera’s aperture setting to make this series, the shutter speed to be constant.

Have the roll developed by the photofinisher. Have the photofinisher read the values of the gray card with the densitometer. If black & white film, have them read the visual values (Yellow filter). The visual value trims the instruments response so it matches the human eye. If the test is preformed using color negative film, the Red reading is the significant value.

The densitometer returns values in density units. This is the language of photo science. The 18% gray card reads the log base 10 value of 18% = log of 18 ÷ 100 =0.18 now 1/0.18 = 5.556 now log base 10 of 5.556 = 0.75. This is the value you are looking for. A spot-on exposure of a gray card provided the developing is spot on, reads 0.75. This value on the film is elevated by the value of the clear film (base fog). Suppose the base fog reads 0.10, then the gray card will read 0.75 + 0.10 =0.85.

Now each f-stop delta (change) elevates or depresses this reading. The delta for 1 f=stop is 0.30. However, this assumes a gamma of 1. Pictorial films with a gamma of 1 are too contrasty. Pictorial films generally have a gamma of 0.8. This being true, the delta for 1 f-stop change is 0.30 X 0.8 = 0.24. In other words a 1 f-stop over exposure will read 0.85 + 0.24 = 0.99, whereas a 1 f-stop under exposure reads 0.85 – 0.24 = 0.61. The delta for 1/3 f-stop is 0.08; the delta for 1/2 f-stop is 0.12.

By now you know this post is filled with gobbledygook, meaning there are lots of pitfalls to run a test on your meter. In closing, a visual trial and error test is likely best as opposed to using instrumentation.