Like SSEBONNE4EVA said, you can narrow things down maybe by feeling the heater hose. Feel both of them and see if either one is hot. If one is hot and the other is cold, that implies that the hot coolant from the engine is not making it through the core. If that is not caused by an air block in the system, you may need a new heater core
If they are both hot (one a bit warmer than the other) that'* normal for a good heater core. That would point you toward a problem with air flow, maybe the air mix actuator that lets air flow through the core into the ducts, or one or more of the flaps not working right.
If they are both cold, or not very hot, again no flow through the core is indicated.