How can I verify a camera’s true ISO, aperture, and shutter values?

ISO is the easiest of these to test properly, but it requires controlled conditions. A standard approach is to photograph a calibrated light source with known output and spectral distribution, ideally recording RAW. Then inspect the RAW data with a tool such as RawDigger and determine the exposure level at which a channel—typically green—reaches saturation. From that boundary exposure value, you can infer the camera’s actual sensitivity and compare it with the displayed ISO.

This is essentially how true sensitivity is characterized in lab testing, so if you want a reliable reference, independent laboratory measurements are often the best source.

Shutter speed can be checked experimentally using known motion, as you suggested. Aperture is harder to verify precisely outside a lab, because the marked f-number may be nominal and real transmission can differ. In practice, published settings are often approximate rather than exact, and ISO is the one most commonly found to differ from the label.