To some, this is a no brainer. But I see many people, LEO or not, failing to ask the proper questions in order to actually get useful answers. Don’t offer up the answer when you ask the question.
Sometimes when I am looking for somebody, I know what vehicle I suspect they are in. When I am asking potential witnesses, I don’t answer the question for them. I wouldn’t ask, “Was he driving a blue Toyota Truck?” Many people won’t think twice and just agree. I always ask, “What kind of car was he in?” I often give vague choices when it comes to vehicle, because many people aren’t “car people.” I purposely leave out what I think the answer is. “Was it a car or a van maybe?” Make them say it’s a truck. Then get into specifics. “What color was it” “Small truck or a big truck?” Don’t ask if he had curly brown hair, it’s too easy to just agree with you.
Plus, most of the “how to tell someone is lying” stuff is based on their body language while trying to fabricate the lie. If you present them with a possible answer, they don’t have to do any fabricating.
This also works the other way. You can offer and answer, that you can easily prove to be false if they go with it. They say, “Yep, that’s it.” Then you bust them on it and explain how lying to a LEO could be a bad thing and maybe they better tell you what you need to know. I find many people, that when caught in the initial lie, end up “fessing up” because they have used up all their creativity for the day.
Don’t let on what you already know. If they think they can give you just enough to get you off their back, but not enough to help you, they might. If they think you already know, they might just clam up.
This works great on your kids too. My wife always asks, “What happened?” when one of the kids is crying or something is broken. And when the kids just stare blankly, she offers a plausible explanation and they usually say, “Yeah, that’s what happened.” I tell her, why did you even ask the question if you were going to give them the answer?
Remember, criminals and kids are often lazy by nature, if you offer them the answer, they’ll gladly take it.