Ok, the way the question is worded they make it clear what the question is, but with some of problems you need to come up with the question. In those cases make sure that a double negative will not give different results if answered by one or other of the people being questioned.
Liar problems are really fairly simple and just want you to make a logic table
| I am talking to / | The liar is / | Answer |
| Peter | Peter | YES |
| Peter | Paul | YES |
| Paul | Peter | NO |
| Paul | Paul | NO |
Logic tables list all the possible conditions and outcomes. Once built they provide lookup tables that let you determine either your current state or states based on your known conditions. In this case since we know the answer to the question is yes we can see we are talking to Peter. If we had been talking to Paul the answer would have been No.