I have a Goodman and regardless of whether I set it to cool or heat no air is blown out from the furnace. The compression turns on and the pilot lite turns on when set to heat, but I don't get anything else. Not sure if it is the blower or main board. How do I test and what may be the problem?

First try turning the thermostat fan control from auto to ON and see if the blower comes on then report back.

Nothing happens

I would start by testing the motor to see if it is the problem. You can use a meter to check to see if power is being supplied to the motor from the circuit board OR you can remove the motor wires from the circiut board and supply the correct power directly to the motor wires from a cheater cord plugged into the correct voltage. Most residential gas furnaces use 120 volts but I would check the label on the furnace to be sure. If you do not know how to use a meter or jumper cord then the testing will best be left to someone who does since you do not want to cause any further damage to the equipment OR yourself bt an electrical shock.

I'm taking it if the blower/motor works for external power that it is the circuit board

Yes it would be the motor power supply if it is located on the board.