Two guys, number One and number Two. You ask number One "Is there a truth-teller among you?". He replies and you have the answer to your question. What is number One and what is number Two?
1 Answer
Answer:
#1 is a liar and #2 is a truth teller
Reason:
Assume that #1 and #2 are truth tellers, the answer to your question is Yes
Assume that #1 is a truth teller and #2 isn't, the answer to your question is Yes
Assume that #1 is a liar and #2 is a truth teller, the answer to your question is No
Assume that #1 and #2 are liars, the answer to your question is Yes
Since you are able to deduce who is who from this single question, the only combination where there is no uncertainty is if you get a No response.