While not always, you may notice an abnormal noise coming from the system, your heating or cooling system running non-stop, or warm air/cool air coming from the vents during the opposite season. Abnormally high utility bills are also a common indicator.
Problems can be caused by a number of different factors. The most common include:
Normal wear and tear associated with age. As your HVAC system is nearing the end of its life cycle you may see an increase in maintenance bills. This is a good indication that it is time to replace your system.
A failed part due to any number of reasons. Routine maintenance on your HVAC system can sometimes head off these types of problems. You can catch them before they become major problems.
Improper or poor maintenance. Lack of maintenance also decreases the efficiency of your system and makes it more costly to run.