Saturation isn't so much the concern as testing the membrane's integrity.
If the microfiltration uses constant transmembrane pressure, that parameter is monitored to detect any fouling or weak fibers.
The saturation threshold can be a degree of fouling or scaling that no longer allows the cartridge to provide enough water downstream.
Fouling on complex modules such as hollow fiber or spiral wound is monitored by capturing the flow rate as it passes through the microfilter and how this affects transmembrane pressure. For simple cartridges, it's the head loss that tells the story. When the flow increases, so does transmembrane pressure or head loss, depending on the module. In determining the upper saturation threshold, you have to think about the flow rate.
In the case of a cartridge, once it has been discarded, you can look at its surface. Fouling may just be on the surface. You can also cut into it and, if the retained particles are colored, observe how far they penetrate. And that gives you an idea of the saturation.
For more complex modules, you could weigh them.