Well you can tell just by looking at the water. When I put the filter in you could tell immediately just by the color of the water and by the feel of it in the shower as it doesn't dry out your skin at all because all of the chlorine has been removed. So when I notice the water isn't as clear and when it starts to dry out your skin that it needs to be changed. It's filtering basically everything and it's easy to trust the manufacturer because it's very obvious that it's doing what it says for the reasons I already stated as well as the water tasting much different.
APEX WH-1020 Whole Home GAC Filter System.Heavy-duty whole home filtration system delivers purified water,Removes chlorine, organic chemicals and bad tastes and foul odors caused by chlorine disinfection chemicals.
apexwaterfilters.com
I think this is the one I have.