verb

• (transitive) To find out definitely; to discover or establish.
  "As soon as we ascertain what the situation is, we can plan how to proceed."
• (transitive, obsolete) To make (someone) certain or confident about something; to inform.
  "Therfore the saide cõmissioners shall mowe say that nowe late during the parlement the King ascertaigned of the saide maliciouse pʳpose of his enemys, willed and desired the lords being then present to shewe their̃ good willes aide and helpe for the saide rescues[…]"
• (intransitive, Yorkshire, dated) To look for something lost.
• (transitive, archaic) To establish, to prove.
  "The two firſt lines of the following book ſeem to aſcertain the true meaning of the concluſion of this, and to prove ſufficiently that by Ωκεανός here, Homer could not poſſibly intend any other than a river."