The saturation luminance is the brightness level above which pixels stop recording unique values and instead record the same maximum value (which is often interpreted as pure white).
You can find the saturation luminance by taking images of increasing brightness until the images start to come out pure white. You can increase brightness either by changing the lighting, or the shutter speed / aperture value.
The saturation luminance is only meaningful however when you also know the luminance at which noise dominates the signal. The ratio of these two figures is your dynamic range.
From your other questions it seems like you're trying to measure the dynamic range of a phone camera. This is going to be very difficult unless you have software to control the camera parameters (namely shutter speed, I assume the aperture is fixed), or have a well calibrated lightsource.