When the lens is focused at infinity, the sensor lies right on the
image-side focal plane, and the corresponding principal plane is one
focal length from that plane, towards the lens body.
There is no simple way to find the object-side principal plane. It's not
on the specs. You may be able to calculate its position if you have the
optical formula of the lens. If you have the actual lens, you may also
measure the position of the object-side focal plane by reversing the
lens and imaging a faraway object. The corresponding principal plane is
again at one focal length from the focal plane.