Actually parents can't tell much better than chance if their kid is lying.
Four-year-olds lie about every two hours. A six-year-old about once an hour. In studies where kids are observed in the home about 96% of the kids lied. Most lie to cover up something they did wrong. Avoidance of punishment is the the primary reason kids lie to their parents.
So what is a parent to do? In the book Nuture Shock, Bo Bronson offers some suggestions
1. Teach your kid the value of honesty even more than lying is wrong. This can be done by offering both immunity and a clear path to good standing with you if you suspect lying. In other words truth will make you happier than hearing the "good news" that the lie represents.
2. Do you value honesty in your own life? Children learn by watching. Are they seeing you handle situations in your life in a truthful way?
3. Teach your child the impact of their behavior on others. Helping kids develop empathy is an important skill. Most younger kids want to please their parents, so make sure they know that telling the truth pleases you more than making you happy by their lie.
By later elementary age, most kids have learned that lying is OK in certain social situations (i.e. white lies to prevent hurting another persons feelings). Lying is both normal and abnormal at the same time. Most kids decrease in their level of lying as they grow older, but about a third of them continue lying a lot. Early intervention is helpful in decreasing the likelihood of this happening.
Remember that the single biggest factor in impacting your child telling the truth is how you as the parent react to the lying. This can affect the behavior for years to come.