Well it’s not just health issues, there’s the arrangement and depth of the creases, callouses, tan lines and stains, cuts and scrapes. If you can map those broadly to character traits (rough and calloused hands imply hard work and industriousness, lots of little injuries imply absent-mindedness, etc), then you can use those character traits to predict the answers to some questions. An absent-minded person is more likely to meet with small misfortunes, for instance.

Combine that with whatever the subject’s questions are, and a bit of basic psychology.