What is the best way to find an electrical leak to ground?

An "electric leak" to ground from a hot wire should trip a breaker or fuse.

If you believe your neutral is going to ground before it reaches the panel, this may be hard to find. I would start by ensuring that your neutral connections or done correctly at each splice.

While doing this, check to ensure that you do not see any bare conductors on your neutral. This may be caused by mishandling or varmints (that would be mice and such). Also check to ensure that device screws (that would be screws used to mount switches, receptacles, and such) have not punctured your neutral. Damage like this can be hard to find because the damage may be so small that when the device is removed, the hole created by the screw may close itself and not be visible.