Yes, but you need to measure the lens’s effective aperture, not just the physical size of the front element. In real multi-element lenses, the marked f-number relates to light transmission through the optical system and the entrance pupil, not simply the front glass diameter.

A practical approach is to use a controlled setup and measure blur or light transmission:

• Use a point light source at a known distance.
• Focus the lens at a known subject distance.
• Measure the size of the out-of-focus highlight (circle of confusion).
• With the exact focal length and focus distance known, you can work backward from depth-of-field/defocus relationships to estimate the working aperture.

An even more direct method is to measure the amount of light passing through the lens under controlled conditions, since effective aperture is fundamentally about transmitted light.

Also, your front-element comparison is not definitive: on complex lenses, the entrance pupil can differ from the physical front element size, and the exact focal length may not be exactly the marked value. So the apparent mismatch doesn’t necessarily mean the lens is “slower” than specified.