You don't need to spend a lot of money on fancy ghost hunting equipment.
EMF levels pull the compass needle away from magnetic true north. If it deviates more than 30 degrees, that usually indicates a "hot spot". In other words, when a ghost is near, the needle spins and can't find north.
Note: Do make sure to rule out the possibility of an electrical source, and know which direction north is. Also, learn to use the compass in a location that is not haunted first, and avoid power lines (both overhead and underground).
You may even want to consider getting a digital compass to work in conjunction with the traditional version, because digital detection is not the same as traditional magnetic.