Student or Learner
In any event, the question she kept turning over in her mind was a simple one: why [the man had perfect fingernails]? Motivation was telling, even when such small things as fingernails were concerned.
(C. Paolini; Inheritance)
What does the 'telling' mean here?
Is it an adjective meaning 'revealing the true nature of a person'?
Yes, Suprunp, I'd say you have the exact meaning of 'telling' as it applies to the context given.