Other tell-tale signs, according to Guy, include your partner being 'dismissive and forgetful' and wanting to know when you'll be home.
READ MORE: • Husbands reaveal the worst things about marriage
He also claimed his girlfriend began "overusing her phone" and "playing the blame game" throughout her alleged infidelity.
Guy went on to explain how he had managed to snare her.
"Being skeptical I started paying more attention," he wrote. "One day I purposely started a fight with her knowing she'd run away but I wanted to know where she would go.
"Sure enough we had a verbal altercation and she ran to him - I followed her car and waited for a few.
"Called and asked where she was, she said she was at her moms ; RIGHT THEN AND THERE I KNEW , she was cheating for sure."
Guy's blog, which was written in April, has been viewed more than 62,000 times and received hundreds of "upvotes".
The warning signs
• Being less interested in sex
• Being dismissive and forgetful
• Wanting to know when you'll be home
• Overusing their phone
• Playing the blame game
The site is known as a place for people to pose difficult and often controversial questions. Last month, a mother sent a word of warning to fellow parents after she found a stream of shocking messages on her teenage daughter's phone.
Lara, a clinical pharmacist, told how she had been tempted to look through her child's phone when she was 13, explaining how "something just didn't feel right with my daughter and her friends".
She allegedly discovered a "stream" of Facebook and text messages between her daughter and the father of one of her school friends, a "40 something year old man".
Describing the impact the ordeal had had on her family, Lara added: "She hated me for a few months. She told me I ruined her life and would barely speak to me. But I knew in my heart what she couldn't know. I knew I had saved her from a predator."