The Mine Detector is used by the Sapper to find mines. This is a great tool to detect mine ambushes near barbed wires.
To use this, click on the Mine Detector Icon or press the Hotkey J and the Sapper will look like holding a long pole, searching from mines. Just let the Sapper walk for a few steps by clicking on the ground. It will reveal small mines. Stop walking near them, because they might explode unto the Sapper. Click on the mine to deactivate it. When deactivated, the mine can be picked up and used later by the Sapper.
The Mine Detector is a great tool especially when going to areaas where barbed wires are installed. Behind every barbed wire, there is always a mine. So always use this before walking or your Commandos might die.