Looks. I can see the dirty side without disturbing the gasket at all due to the convenient arrangement and orientation on both of our Subaru's. The filter for the Forester looked good at 50k miles, but I changed it anyway. The filter sits vertically in the housing, and the bottom pleats were darker/dirter, but overall the filter looked just fine cleanliness-wise and structurally. I won't change the filter next time until it looks to need it, hopefully after another 50k miles or more.