Temperature gauge reading properly but no heat coming to cabin

Coolant levels are fine. What is the usual problem (something I could see with the naked eye) and is there a checklist I should be going by in order of first to last.

Most common are air bubbles in cooling system or restricted heater core. If heater inlet and outlet hoses are not both hot, it's likely one of those. If they are both hot, then issue is more likely in the dash - service manual procedures are used to diagnose this. Tom