A proper breeder should be comfortable discussing this with you, and providing information on the dog's family history. These are great dogs for city or country. The breed is prone to early onset of heart trouble, which we have been fortunate to avoid so far. One was bad enough to need surgery. Early onset of deafness or hearing trouble is being noticed by people who train them.
Back trouble is another problem.
One of ours has invertebral disk disease. Hip displasia should be checked as well. One of ours has mild hip displasia. There can also be eye problems. Cavaliers are beautiful, happy, extremely loving little dogs, with an enthusiasm for life.