Warm the engine up, stop the engine.

Then disconnect the exit side of the heater core house

Connect a length of hose into a large jug (a few gallons)

With the rad cap open (no pressure in system), measure the flow rate through the heater core... if the flow out is not greatly fast, then:

A) try back flush the heater core (disconnect other hose, feed hot water through it).

B) failing that, replace the heater core.

Before you do B), make sure that there is not a vacuum operated door that is closed or not operating properly causing no hot air to go from the core to the car interior.