Many drivers realize that they need to change the engine air filter as soon as it looks dirty to the naked eye. You might notice that there's a serious clog somewhere in the filter paper, and this will surely tip you off to the fact that you need a new one. The problem is that so many automakers now opt for completely sealed filters, which are much harder to visually inspect.
If you can't tell merely by looking at it, then take note of any dramatic reduction in acceleration or weird noises that your fan is making. These can be signs of poor airflow. Drivers of vehicles with cabin air filters usually know right away when they don't have nearly as much air coming in as they used to, which is almost a sure sign of a clog.
Head over to Bell Ford next time you're in Adrian and need your filters changed.