Unfortunately there aren’t simple methods to check the saturation of the AC. The ways to be followed can be partially two: the first is to analyze the air/gas treated after the Activated carbon filter; the second one is the take periodically a sample of the Activated Carbone and check the residual activity and V.O.C. loading.
Το check “optically” when the Activated Carbon is saturated is possible to put a layer of Activated Alumina Impregnated with KMnO4 on top of the AC.
The Potassium Permanganate will react with the V.O.C. that eventually begins art to leaks out from the activated carbon changing the surface color of the media from bright purple to light brown and then grey.
This solution is simple and give an indication about the saturation of the activated carbon, but is not reliable and accurate.