Here are two examples from my bikes:
Note the Bontrager ANT+ sensor on the fork. You can barely see the magnet in front.
The Zipp 808 front isn't a directional wheel, so I put a magnet on both sides. The sensor is on the left fork arm. This is the right side. So, if I happen to put the wheel on in the opposite direction, there will be a magnet in place.
