An idea of depth a crack may have penetrated into metal or danger of breaking can be done with magnetic test. Use V - Shaped or Horseshoe shape on each side of crack area with magnetic pole on each side. Sparingly sprinkle iron fillings on the crack. Iron fillings will stand on end if crack runs deep enough to cause a break.
It would seem DC battery and insulated wire wrapped around one leg of V- Shaped magnetic metal (electro-magnet) would work.
