There are several the symptoms that your heater core is malfunctioning or leaking. The most obvious sign is that vapor is being produce inside the car causing the front windshield to fog up. Another symptom is that your car simply does not produce any heat during from the ventilation system. In either case, you either have a leaking or blocked hose or a cracked core and will require you to repair the problem.
I read somewhere adding a stop-leak additive to your cooling system will fix the issue. But this is temporary short-term solution and you will eventually need to replace the part. Personally I would not do this because these additives usually don't work too well in the core. Plus the problem may be due to a blocked hose.Hot antifreeze circulates throughout the cooling system radiator hoses and heater. Both heating pipes which are attached to the heater core under the hood. If the heating pipes are not hot, inspect the radiator hoses are not obstructed internal cooling system.
Testing the pressure of the cooling system is the next step in detecting a leak. A pressure gauge is connected to the radiator and air is pumped in to increase the pressure of the system. The pressure will drop if there is a leak in the system. Either hoses or the heater core needs replacing.
If the pressure did not drop and you did not experience vapor, you may have a blocked hose. Usually if your car does not produce heat, your car could have this problem. Sometimes a radiator flush will dislodge the gunk causing the fluid to run through the heater again. But you may have to flush the core backwards to remove the blockage.
Replacing the part is difficult and is well worth the money to have an experienced mechanic install a new one for you. The location of the core is underneath the dash making it a real pain to repair. Since it is a labor intensive job, expect the repair to run several hundred dollars. It is best to call several mechanics to get an upfront estimate especially since you already know what the problem is.