heater not heating

i am having a similar issue with a 1996 suburban 1500. the air comes out strong but it never gets hot. ive been looking online for fixes and so far ive come across the heater core being bad. ive replaced heater cores before but its not my favorite thing to do. but i just want to be sure that that is actually the problem before i tear into it.

i though maybe the thermostat is bad or maybe the heater core could just be plugged. Im not really sure. could you guys help me out?