The Collaborative International Dictionary of English v.0.48:

Questioner \Ques"tion*er\, n.
   One who asks questions; an inquirer. "Little time for idle
   questioners." --Tennyson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

questioner
    n 1: someone who asks a question [syn: inquirer, enquirer,
         questioner, querier, asker]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "questioner":
   Paul Pry, Peeping Tom, asker, busybody, catechist,
   cross-interrogator, cross-questioner, detective, eavesdropper,
   examiner, gossip, inquirer, inquisitionist, inquisitive,
   inquisitor, interlocutor, interpellator, interrogator,
   interrogatrix, interviewer, nosy Parker, opinion-sampler, poller,
   pollster, prober, pry, querier, querist, questionist, quidnunc,
   quizzer, rubberneck, rubbernecker, sampler, scopophiliac,
   secret agent, sightseer, snoop, snooper, voyeur, yenta