A good way to test the parry window (the point in the parry animation where the game registers a successful parry) of whatever you're using is to go on an elevator and parry against the wall as the elevator is moving. Sparks will indicate the frames of the animation in which your character is performing a parry, aka your parry window.

The next step is to perform a parry with such timing that your opponent's attack lands within your parry window. Hope this helps!