If one has fully repented of a sin can you tell people that you have never committed that sin or would that be untruthful? For example if I smoked and fully repented of that sin and someone asks me if I have ever smoked can I say no or should I indirectly answer the question with something philosophical?

Anyone who really has a testimony and a little understanding of the Atonement performed by the Savior Jesus Christ knows that for all practical purposes past sins "die" once the repentance process is concluded and the person is forgiven by the Lord.

Once divine forgiveness is obtained, whatever a person did or didn't do in life becomes nobody else's business, and whoever asks a question about past sins without being properly authorized to do so is in reality being more curious than he or she should be.

Generally, only a person's bishop and a spouse have the right to know everything about one's past.  In second place would come parents (if the problem requires their knowing about it,) and next in line might come a certified therapist.