How can I accurately locate bricks covered in stucco or plaster?
While this doesn't answer the question directly, I question the need to know where the bricks are when hanging items indoors.  Unless the load on the anchor is going to be axial rather than lateral (ie, pulling rather than hanging), the bricks above and below should still provide support for the anchor even if you are in a mortar line.

Alternate Solution

Perhaps using a different anchor style would solve your problem.  One that flares more when tightened, to better embed itself in the mortar.  Combine this with epoxy to further increase the strength of the anchor.