With the engine OFF, make a jumper wire from the battery (+) and touch the other end to the PTO clutch connector a few times. You should be able to hear it click on and off. If you can, the clutch is good. Remove the jumper and reconnect the original connector to the PTO clutch.
The PTO clutch could be wired in series with several "Safety Switches". One most common is on the seat. There has to be pressure on the seat to engage the PTO clutch (blades). There could be others. The switches do go bad and fail to make contact. Check your manual for a wiring diagram and what has to occur before engaging the PTO. Some won't engage the PTO if the mower is in reverse but will go into reverse with the PTO on. There may be a switch on the clutch or brake as well. Read you manual.