With the heater controls off and the heater blower fan off, bring the engine up to operating temperature. Shut off the engine and feel the two hoses that go to and from the heater core. Spread two fingers and contact the hose with the more sensitive, thin skin between. You will find the heater hoses on the rear passenger side of the engine compartment. With a good heater core and no coolant flow problems (like low coolant level or a big air bubble trapped in your engine) both hoses should feel hot. When the fan is on high and the heater is blowing a bucketload of hot air, you might find the return hose from the heater core noticeably cooler than the feed hose. But it should still be pretty warm. If just one hose is hot when the heater blower fan has not been running, the problem is likely insufficient coolant flow through the heater core rather than air flow through the ducts. This could be a plugged core or that the core is not getting enough coolant from the engine. If both hoses are nice and hot, start to look at the flaps and ducts and the vacuum and electrical circuits that control them.
BTW, low coolant level or trapped air often creates a sometimes hot core depending on engine speed, angle off level, and other factors.
I would not recommend doing this with the engine running. The metal fan on the alternator and the alternator belt and pulley can quickly cause a problem a lot more serious than lousy heat.