-set the heater to maximum hot
-warm the car up
-turn off the motor
-open the hood
-feel the heater hoses

if they're hot, the heater core is working correctly (see air mixture instructions, below)

If they're cold, the core is probably plugged up or the heater valve isn't adjusted properly or is disconnected from the control cable

Air mixture instructions:
If the heater hoses were hot, the air mixture door on you car's heater module is not adjusted properly or not working at all. (on my 1991 crx si, the mounting point of the air mixture control cable at the heater control dial broke loose; I drilled a 1/8" hole and zip tied the cable back into position which fixed my heater problem.)

The other end of the air mixture cable is on the passenger side near where their left foot rests. Check to see if it's connected to the door; adjust it by setting the heater to max heat, loosening the clip holding the cable, and moving the cable so the door is providing air from heater core only, then tighten the clip.
(note: the manual says to check the heater valve cable adjustment after adjusting the air mixture cable.)