The Collaborative International Dictionary of English v.0.48:

Ascertain \As`cer*tain"\, v. t. [imp. & p. p. Ascertained; p.
   pr. & vb. n. Ascertaining.] [OF. acertener; a (L. ad) +
   certain. See Certain.]
   1. To render (a person) certain; to cause to feel certain; to
      make confident; to assure; to apprise. [Obs.]
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            When the blessed Virgin was so ascertained. --Jer.
                                                  Taylor.
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            Muncer assured them that the design was approved of
            by Heaven, and that the Almighty had in a dream
            ascertained him of its effects.       --Robertson.
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   2. To make (a thing) certain to the mind; to free from
      obscurity, doubt, or change; to make sure of; to fix; to
      determine. [Archaic]
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            The divine law . . . ascertaineth the truth.
                                                  --Hooker.
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            The very deferring [of his execution] shall increase
            and ascertain the condemnation.       --Jer. Taylor.
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            The ministry, in order to ascertain a majority . . .
            persuaded the queen to create twelve new peers.
                                                  --Smollett.
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            The mildness and precision of their laws ascertained
            the rule and measure of taxation.     --Gibbon.
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   3. To find out or learn for a certainty, by trial,
      examination, or experiment; to get to know; as, to
      ascertain the weight of a commodity, or the purity of a
      metal.
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            He was there only for the purpose of ascertaining
            whether a descent on England was practicable.
                                                  --Macaulay.
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WordNet (r) 3.0 (2006):

ascertain
    v 1: establish after a calculation, investigation, experiment,
         survey, or study; "find the product of two numbers"; "The
         physicist who found the elusive particle won the Nobel
         Prize" [syn: determine, find, find out, ascertain]
    2: be careful or certain to do something; make certain of
       something; "He verified that the valves were closed"; "See
       that the curtains are closed"; "control the quality of the
       product" [syn: see, check, insure, see to it,
       ensure, control, ascertain, assure]
    3: find out, learn, or determine with certainty, usually by
       making an inquiry or other effort; "I want to see whether she
       speaks French"; "See whether it works"; "find out if he
       speaks Russian"; "Check whether the train leaves on time"
       [syn: determine, check, find out, see, ascertain,
       watch, learn]
    4: learn or discover with certainty

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "ascertain":
   afford proof of, appraise, ask, assure, be informed,
   become acquainted with, bring home to, broaden the mind, catch on,
   certify, cinch, clear up, clinch, consider, contemplate,
   cram the mind, decide, demonstrate, determine, discover,
   dismiss all doubt, ensure, establish, find, find out,
   find out about, fix, follow, follow from, gain knowledge, get,
   get at, get hold of, have a case, hear, hold good, hold water,
   inquire, inspect, insure, interrogate, learn, learn about,
   load the mind, make a decision, make certain, make good,
   make no doubt, make no mistake, make out, make sure, make sure of,
   nail down, observe, pick up information, prove, prove to be,
   prove true, query, question, reassure, remove all doubt, resolve,
   see, see that, see to it, set at rest, settle, settle the matter,
   show, sort out, study, survey, tumble, unearth, view, weigh