Mostly you can tell because it’s not doing as good a job at filtering - even with the fan all the way up to 11.
People have put in pre-filters to extend the life - those will catch a lot of the larger bits. Something like this, cut in half: https://www.amazon.com/gp/product/B07JZ3QBX9/ or read down this thread for both other pre-filter options, and a cleaning one: