Chevy Silverado Heater Not Working?

I have a 2001 Chevy Silverado. The AC works fine, but when I turn on the heat it blows only lukewarm air at best. If the truck is hot it will come out warm for a minute when I first turn it on, but it does not last. It is hot when the blower is off. I have not checked anything as of yet. I was told the actuator may be stuck. Is there a way to determine if it is the heater core or the actuator? Any tips would be greatly appreciated.