All kids lie at some point, and we can't always tell when they're doing it (those little buggers). There are two things, however, you can say to your children to get them to be honest.
Eric Barker cites findings from NurtureShock: New Thinking About Children, which says that children (at least young ones) lie to try to please you or make you happy. So the first strategy to getting the truth out of them is to tell them you'll be really happy if they tell you the truth:
What really works is to tell the child, "I will not be upset with you if you peeked, and if you tell the truth, I will be really happy." This is an offer of both immunity and a clear route back to good standing. Talwar explained this latest finding: "Young kids are lying to make you happy — trying to please you." So telling kids that the truth will make a parent happy challenges the kid's original thought that hearing good news — not the truth — is what will please the parent.
The second thing to say can cut down lying by 25 per cent: "I'm about to ask you a question. But before I do that, will you promise to tell the truth?" (Hopefully the kid will say "yes.")
Check out the full post on Barking Up the Wrong Tree for more parenting tips and tricks.
Good Parenting Skills: 7 Research-Backed Ways to Raise Kids Right [Barking Up the Wrong Tree]