I always have to Google which of my bike pedals is reverse threaded. It's a bit embarrassing, as I always think I should be able to figure it out from the whole angular momentum - torque thing.
With the crank in the position closest to the front tyre (horizontal and forward), put your allen key in the socket while aimed at 12 o'clock (ie, upward) and rotate toward the rear of the bike.
The rule is basically: rotate against the direction of the crank's rotation to undo.