Return a difficult question to the person who asked it. Ask his opinion on the matter. After his answer, or you will agree with his words, or to express their views, which could take shape in the process of listening to the interlocutor. Often people just want to show off, giving a smart and lengthy reply, so your call forwarding can be very useful.
Tell me exactly where a person can find a complete and comprehensive information on the subject. Can mention that doubt in some respects, so for a more precise answer is better to use a proven source. This positively characterize you as an honest man who does not want to do is certainly questionable statements.
Ask a little time to think about the answer, if possible. During this time, look for information in books or make an inquiry online. Read the found materials and slender specify detailed answer. It is always useful to acquire new knowledge and to show that you can, and do not be lazy to do it. A person who grows and reveals its potential is always appreciated more than an employee who does not accept anything new and does not develop.
And finally, if you do not see the other possibility, admit that you do not know the exact answer to a question for you. But never give false information, trouble in this case will surpass all your expectations. Trust is very hard to win, but so easy to lose. If you do not believe in their words, mention this, then your answer will not be perceived as a fraud.