Sally*, a lawyer in her 30s, was getting suspicious of her new boyfriend of several weeks. Rick*, a handsome air steward whom she'd met on a business trip, was being evasive and constantly rejecting her phone calls.

When he did return them, he spoke in hushed tones and gave excuses such as that he had been busy babysitting his niece. She wanted to get to the bottom of things.

So, barely a month after meeting Rick, she paid a four-figure sum for a private investigator (PI) to follow him around for a week and discovered that the man she'd been dating was actually married. He was ignoring her calls so that his wife wouldn't get suspicious, returning them only when he was alone.

And all that time "babysitting" his niece? He'd been fetching his daughter from school.

Sally's story is just one of several that I heard when I interviewed four local PIs about similar cases. While it's common knowledge that PIs are hired to investigate cheating husbands, more unmarried women like Sally are now paying them big bucks to tail boyfriends - sometimes way before marriage is on the cards.

Those in the industry call these cases "premarital background checks". For some women, these checks are becoming an essential step before taking the relationship to the next level. Met the parents? Not good enough. You have to pass the "PI test".

"It used to be that concerned parents would hire PIs to check on their future sons-in-law. But over the past two years, I've seen more girlfriends and brides-to-be coming straight to us," says Richard Koh, managing director at Glen Iris Services, who has been in the PI business for 14 years. Before 2010, he'd never even heard of such cases. Last year, he had three and, this year, he has already resolved one.

Over at APAC Investigation and Consultancy, operations manager Raymond Lim says that he began taking on such cases two years ago. Now, he gets an average of one a month.

Typically, women who hire PIs to check on their boyfriends are in their 20s and 30s, in white-collar jobs (think administrators and bankers). They're usually engaged or in serious relationships, and want to check if their boyfriends are faithful and have been truthful about their jobs or financial status.

A small number of clients are also divorcees who want to take precautions before getting into another relationship.

Mr S.M. Jegan, managing director of Kokusai Security (also behind the popular catchcheatingspouse.org.sg website), says about 40 per cent of his caseload are pre-marital checks. He adds that he's been taking on such cases since the 1980s. Whether the women are young or old, their reasons for seeking him are the same.

"They want to know if their boyfriend is sincere, loyal and truthful. And they want to ensure that they're not sharing him with another woman," he says.