LEAVING the house early, staying late at work and unexplained condoms in the car.

Not to mention the old classic — lipstick on the collar.

They are all signs your partner is cheating, according to Tom Martin, California-based private investigator, reports The Sun.

Tom, president of Martin Investigative Services, has now written a book detailing the 20 clues that most likely indicate infidelity.

But he warned people not to panic if they had spotted a few telltale signs that their partner was playing away.

In his book, Seeing Life Through Private Eyes, Martin said spotting one or two warning signs was not a problem.

But four or more was usually a reason to worry, he said.

“If you have three or four, you have a problem,” he told theIndependent.

And he waned the internet and smartphones had hugely changed how people cheat — enabling easy connections.

“The game has changed drastically,” he said. “And technology is 100 per cent the reason why.”

Her are the 20 signs people are cheating:

• Changing habits

• Leaving the house early and returning late

• Business trips

• Holiday or family event absences

• Excessive overtime

• Unexplained spending

• Secret accounts (Instagram, email)

• Hidden credit card bills

• Extra grooming

• Scent of someone else (typical lipstick on the collar)

• Buying gifts you haven’t seen

• Unexplained items — condoms in the car are a giveaway

• Joining a gym

• Missed calls from an unknown number

• Coded or secretive text messages

• Less sex

• Evasiveness or defensiveness

• Obvious lies

• Prickliness or short temper

• Hate surprise visits (your unexpected arrival may come at an inopportune time for a cheater)