Much depends on the make and model of the furnace. Some new systems have some very odd features.
But as a simple test, turn off the power to the furnace and go to the furnace control board. At the control board, disconnect the Fan wire coming from the thermostat to the control board, usually connected to the G terminal. Also check the control board to see what motor connections are available. Usually there will be spade connections for Heat and Cool. All other blower motor leads will be "parked" at unused space connections for safety.
Possibly there is a Fan spade connection at the control board and it is connected to the slower speed tap of the motor. Without knowing the furnace these are just guesses. You may not have a multispeed PSC blower motor. Check the schematic on the back of the blower compartment door for more information.