Testing the condition of bronze turnbuckles
The BEST inspection is done by taking the rigging to one who has the magnetic field (Magnaflux®) analysis equipment. The next best is to remove the rigging and use a specialized dye penetrant, also from Magnaflux. I would NOT do this on the boat .... a degreaser, a dye, and a developer. Macroscopic cracks due to embrittlement failure (cold working) will show as faint red lines on the developer. I think West Marine also sells the dye system. OR... go to a Magnaflux distributor, be super polite, etc. ... and they just might give you a "freebee" small sample/demo kit that would be enough to do a moderate sized sailboat.
Otherwise, simply clean (but dont polish) the terminal ends and inspect carefully by eyeball or with a magnifying glass, etc. ... for visible "teeny" cracks (not deformations due to swaging), especially those running longitudinally (lengthwise) on the fitting or those across the end from where the cable inserts to the outside surface of the connection. Hope this helps.