Saturation luminance is the scene brightness above which the sensor no longer records distinct higher values—pixels just clip at the maximum value (white).

A practical way to find it is to photograph a target while steadily increasing exposure or illumination:

1. Keep the scene and camera fixed.
2. Take a series of shots with increasing brightness, either by increasing light on the subject or by increasing exposure (longer shutter time, wider aperture, etc.).
3. Examine the image data. The saturation point is where further increases no longer produce higher recorded pixel values and the brightest areas become clipped/pure white.

That gives you the upper limit of recordable luminance.

For dynamic range, this number alone is not enough. You also need the lower limit: the luminance where noise becomes stronger than the useful signal. The ratio between the saturation luminance and that noise-floor luminance is the camera’s dynamic range.

In practice, this can be difficult on phones or heavily automated cameras unless you can control exposure settings reliably (especially shutter speed and aperture/ISO where available).