As with any major appliance, the best recommendation to start with is to follow the manufacturers instructions. If the owners manual is unavailable, you should be able to Google the appliance name and model number to get what you need. If you are still unable to locate any instructions, you can always remove the filter and inspect it for debris. If it looks plugged, clean or replace it. If you are not able to determine if it is worn out, reinstall it and check it again in a couple of weeks and reevaluate its condition. If it looks the same, let it go a little longer this time before inspecting it again. Keep a log of your inspection dates until you can determine that it needs to be replaced. As a general rule for furnace filters, if you can't see the dirt, it doesn't need to be replaced.