Get yourself a meter that can measure from 24 volts to 240 volts, and has an ohmeter, or even a continuity tester. Follow the wires from the fan back through the entire circuit, through the controls looking for the device, usually a relay, to determine what controls it.
The relay contacts may be pitted causing them to stick or the motion of the contacts may be restricted some how from moving. Or the relay is fine and it is because of a reason in the control circuit.
The relay will be wired through control devices, such as high/low limit switches located on the heat box of the furnace, telling when the fan is to be on or off.
Check your manual , there usually is a "Sequence of Operation" list, explaining exactly which switch operates at which condition, from turn up the thermostat, until the unit cools down.
But if you do need to call in a technician, rest assured you are not giving him a "boat" payment. Techs usually make a decent pay, but the knowledge they have does not come easy.
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