How do you locate a ground in home alarm wire?

You can use an ohmmeter or continuity tester. Connect one lead to ground and the other to each of your sensor wires. When you have a ground on the wire being tested, your meter will indicate continuity. Make sure the sensor wires are de-energized when testing with these methods, this may require you to disconnect the system power and unplug the backup battery. If you already know which wire is grounded, you'll need a transmitting/receiving device such as a circuit tracer or short tracer. This device will allow you to follow along a wire and detect approximately where the problem is.