Most people don't pay any attention to their air conditioning system until it isn't working. If you want to save money, you need to pay attention to your system when it IS working.
In order to know when your air conditioner is not working normally; you need to pay attention to it when it IS working well. Only then will you know what "normal" is.
Here are several easy steps you can do so you have a baseline for your system:
1. Listen to your air conditioner. Notice what it sounds like when it is running. Listen to how it sounds when it turns on and off.
2. Time your system. How long is it on and how long is it off. Notice how the timing changes based on the "heat load." Heat load is not just how hot it is outside. Heat load can also be affected by running major appliances (especially the oven) and how many people are in the house.
3. Discover where your air conditioning system drains off the water created by running an air conditioner (condensate water.) If your house is cool and it is hot outside, the system will be creating water that will be draining off. If you don't see the condensate coming out where it normally does, then it is coming out someplace where you don't want it to come out.
4. Put your hand over the unit (about a foot above the fan) while it is running. How warm is the air that is coming out? It should be warm, not cool.
If you pay attention to the above four items, then you will notice if there is a change or not.
As with any repair, early detection reduces the damage and therefore the cost of returning your system to normal operation.