Definitions of telling:

• noun:   informing by words
• noun:   an act of narration
• noun:   disclosing information or giving evidence about another
• adjective:   disclosing unintentionally

  Example: "A telling smile"

• adjective:   producing a strong effect

  Example: "A telling gesture"

• adjective:   powerfully persuasive

  Example: "A telling presentation"