Clothes take more than one cycle
If a normal load needs two runs, lint is likely blocking airflow somewhere in the vent.
The dryer or room gets very hot
Trapped hot air has nowhere to go. A dryer that's hot to the touch on top is a warning sign.
Burning or musty smell
A burning smell means stop using the dryer. A musty smell can mean moisture is trapped in the vent.
The outside flap barely opens
Run the dryer and check the exterior vent hood. Weak airflow or a stuck flap points to a blockage or a bird nest.
What you can do today
Clean the lint screen every load, wash it with soap monthly to remove fabric softener film, and keep the area behind the dryer clear.
General information only, not legal or professional advice.