It's a skill he developed during 40 years of pulling over speeders and interrogating murderers.
But Rush knows that really good liars won't always give themselves away with shifty eyes and fidgeting fingers. That's when he dispenses with instinct and leans on his other speciality.
Rush, 57, estimates he has given 10,000 lie-detection tests during his 15 years as a certified polygraph examiner. Over that span he's heard some incredible secrets.
"Sometimes I feel like a priest," Rush said.
Rush got his start in law enforcement as a dispatcher for the Georgia State Patrol, where he eventually graduated to trooper. He later went to another academy to train as a special agent for the Georgia Bureau of Investigation.
The 10 weeks of polygraph school, though, were the hardest, Rush said.
"There's a lot to learn physiologically and mentally," he said.
The basic premise of a polygraph test is to ask a series of "yes or no" questions, then measure a person's physical reaction. Two straps around the chest gauge a person's breathing pattern; a blood pressure cuff registers heart rate; and two metal sensors on the fingers monitor sweat. Four long pins write the body's signals in ink on a long chart that rolls 1 inch per minute.
The whole package fits into a bulky silver briefcase about 3 feet long.
Rush works for the Columbia County Sheriff's Office now, but he also administers tests privately through his business, CSRA Polygraph Services.
The former typically involves verifying a witness's veracity or finding the truth about an armed robbery. The latter often focuses on questions of infidelity.
But sometimes there's no need for a test at all. Just the prospect of taking a polygraph test can make some people squeal.
Rush recalled a man accused of having sex with a 13-year-old girl. He denied it up until a pretest interview with Rush, at which point he confessed not only to having sex with the girl but also to molesting many more children in other states.
That was an easy day for Rush, but usually what he's after is proof that a person is being dishonest. If the test indicates deceit, a formal interrogation without the machine will follow in another setting.
Rush is commonly asked whether the polygraph exam can be beaten. Technically, yes. But Rush qualifies that answer. Every exam begins with a thorough interview designed to make the results as airtight as possible.
He asks people whether they got a good night's rest, whether they have certain medical conditions, whether they are under the influence of drugs or alcohol. That way no one can say they failed because they were tired.
He reads every question that will be asked during the exam to eliminate any surprises. Rush also explains exactly what's going to happen during the exam and asks whether there are any questions.
"I try to take away every excuse," Rush said.
One of the most common excuses is nervousness. That doesn't fly with Rush. Taking a polygraph exam makes most people nervous, even the innocent ones. If nerves made people fail, "then no one would pass," he said.
Apart from excuses, there are other ways people try to cheat the system. Taking drugs beforehand, for instance, will lower the heart beat and lower the spike on the machine when a lie is told.
Rush personally witnessed the downside of this method with a man whose mother gave him a muscle relaxer before the exam.
"He couldn't finish his sentences," said Rush, mimicking the man's droopy eyelids and nodding head.