The method used to level a pendulum clock depends on whether the clock is free-standing or mounted on a wall. A wall clock needs only a small shift. A free-standing clock needs a shim to lift one of its sides.
To determine if a pendulum clock is level, listen to its sound. If the clock makes an even "tick-tock, tick-tock" without a longer pause between, it is level and should run correctly. If the clock sounds more like "tick-tock, pause, tick-tock," it needs to be leveled.
To level a wall-mounted pendulum clock, stop the pendulum, and let it hang until it is completely still. The pendulum should hang straight, in line with the body of the clock. If it is not hanging straight, shift the position of the clock on the wall until the pendulum hangs straight.
To level a free-standing pendulum clock, lift one side of the clock slightly, and listen. If the sound of the clock evens out, place a shim under the side of the clock that has been lifted. If the pause in the sound becomes worse, lift the other side of the clock, and place the shim on that side. Only small adjustments should be needed.