Some people hire private investigators to catch their cheating partners.

But this woman didn't have to do a thing except watch activity levels on her synched exercise gadget "spike" at 4am.

Jane Slater, an NFL sports reporter in the US, revealed she caught her now-ex boyfriend out after they paired their FitBit devices – a gadget he purchased for her last Christmas.

Slater was was very appreciative of the gift — a device that would later expose his cheating antics.

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The pair linked their FitBits, and for a while all was well as they "motivated" each other to keep on track, news.com.au reports.

But being able to monitor her partner's movements meant Ms Slater was able to catch him out when she noticed something suspicious about his increased energy levels.

In a Twitter conversation with fellow NFL reporter Albert Breer, where the pair discussed exercise gifts, Ms Slater wrote about her FitBit: "Didn't hate it until he was unaccounted for at 4am and his physical activity levels were spiking on the app.

"Wish the story wasn't real," she posted.

For those thinking her ex was just having a late-night or early morning gym session, Ms Slater was quick to clarify: "Spoiler alert: he was not enrolled in an Orange Theory class at 4am."

Since sharing her story on Thursday, the post has since gone viral with nearly 500,000 likes and thousands of comments from shocked Twitter users.

"Of all the ways to get caught that has to be top 5 as most unique," one person tweeted.

"Every female who reads this is about to get their man a fit bit," another joked.

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It appeared some were already onto it.

"Just got my boyfriend and I fitbits for Christmas," one woman wrote.

However, others thought Ms Slater was "jumping to conclusions".

"Maybe he was a former three year high school football letterman like me who rises at 4am every morning to run routes in cleats," one man wrote.

Some shared similar experiences on how a device also led them to their partner's cheating behaviours.

"Jane! My ex husband got caught in an affair because he was wearing the training watch and heartbeat monitor my mom bought him for Christmas – while 'running'. It was the under-one-minute heart rate spike that confirmed things for me!'

Ms Slater had also revealed a time when she was dating a guy who came home drunk with another girl, forgetting she was spending the night.

"We were set to go to church the next morning and I stayed in to get sleep. I could write a book," she said.

"My girlfriend once said, "One day we will laugh about this", as I sobbed uncontrollably in the car. I have now many times bahahaha."