There are 3 things which usually indicates that you have a vents which needs serviced and / or cleaned.
1. Increased drying times are one of the best indicators of a dirty or clogged vent. If it takes more than one cycle to dry a load of clothes, especially jeans and towels, cleaning is most likely in order.
2. If your dryer gets hot on the top of the cabinet during a drying cycle it usually means that the air flow which helps keep the dryer cool is restricted. This can be a broken vent pipe in the wall or ceiling or a clogged vent. Also, if you open the dryer while it is operating you will find the inside of the drum which turns extremely hot.
3. Sometimes you will notice that your clean clothes come out of the dryer smelling musty .