1. It takes more than one cycle to dry your clothes. Clothes dryers evaporate the water from wet clothing by blowing hot air past them while they tumble inside a spinning drum. Heat is provided by an electrical heating element or gas burner. An obstructed vent pipe prevents the airborne water vapor from escaping your dryer, which lengthens the drying time and leaves your clothes wet at the end of a cycle.
2. You notice a burning smell. Lint and fabric fuzz contain cotton and polyester, which are highly flammable. It does not take very much heat to ignite dryer lint. If you notice a burning smell when operating your dryer, shut it off and contact an appliance repairman immediately.
3. Your clothes are piping hot at the end of a load. An excessive buildup of lint restricts the amount of heat and water vapor that can exhaust through the vent pipe from the dryer, leading to accumulation of heat energy within the machine. You'll know this is happening by observing if your clothes are excessively hot to the touch after they come out of the dryer.
4. Your dryer shows indications of overheating. The tell-tale sign that your dryer is too hot is damage to your clothes and a unit that is excessively hot to the touch while operating.
5. You haven't cleaned out your vents in over a year. Obstructed dryer vent pipes are the most commonly neglected home maintenance item for good reason, most people don't think about their laundry room very often. For a typical household, a yearly cleaning is more than adequate while larger households should expect more frequent cleanings.