Hannah felt something was amiss when her fiance kept cancelling date nights due to "overtime work".

After a few months of suspicion, the 30-year-old sales executive engaged a private investigator, who learnt that the man had been spending nights with other women, including an actress, at various hotels.

It was a devastating discovery for Ms Hannah (not her real name), but sadly such cases are not new. Indeed, suspicions among loved ones seem to be on the rise.

Private eyes told The Sunday Times that they are getting more requests from clients who want to check on boyfriends or girlfriends, even before they say "I do".

These "love detectives" saw about 30 to 60 client requests for checks on the dating history of their boyfriends and girlfriends last year, a 10 to 50 per cent jump from five years ago. They have seen a 10 to 30 per cent increase in the number of clients - from housewives to lawyers - checking on their partners from 2012 to last year.

Private eye James Loh, who has been in the business for 16 years, said he handles around 300 matrimonial and family cases a year, 10 to 15 per cent more than five years ago. About 40 of those cases are checks on boyfriends and girlfriends.

"More young people want to consider carefully before jumping into a marriage. They don't want to end up regretting," said Mr Loh, the managing director of International Investigators. He added that there is now an equal number of cases of women and men cheating on their partners - a stark change from five years ago, when men were the main offenders.

Infidelity cases form the bulk of the workload for private investigation agencies, ahead of tracking missing persons and monitoring employee movements. It can cost about $800 to more than $10,000 to hire a private eye, depending on the number of hours agreed upon and whether there are overseas assignments involved.

Investigators find that in 90 per cent of the cases they handle, clients' suspicions about their partners are borne out.

After discovering that their partners have been cheating, many clients have broken down in tears, according to the eight private eyes interviewed. But a few remain eerily calm.

"Clients come to us when they suspect that something is wrong," said Mr Loh, who made a career switch from marketing in 2002.

"It is mental torture for them. They want proof that their partners are cheating."

His firm employs eight investigators, comprising former police and military officers as well as fresh university and polytechnic graduates. He has even received applications from MBA graduates.

But it is not an easy job as it sometimes involves dealing with physical threats, said one investigator who declined to be named.

"If you trail too closely, the target might notice you," he said. "The job also involves a lot of waiting. Some newbies can't take the long hours and they quit after a few days."

Before tailing a target, an investigator maps out the places he or she frequents, including checking the number of exits, and studies the surrounding roads and traffic conditions. Travelling overseas to trail a suspected cheating spouse for days or even weeks is part of the job.

"You are often required to travel at short notice," the investigator said. "Over there, you have to navigate around unfamiliar places discreetly and often with a language barrier."

Their work is complete when they gather good photo and video evidence of individuals hugging or kissing their lovers, or spending the night at a hotel or an apartment.

Getting injured is also part and parcel of the job, with many investigators reporting bruises, scrapes and sprains in pursuit of a suspected cheat.

But some have suffered far more serious injuries. SG Investigators founder David Kang, who has been in the business for more than 15 years, fractured his pelvis last year after his motorbike skidded as he was trailing a target.

"There are real risks. The actual job is different from the movies," said Mr Kang, 54, whose agency has nine investigators, including a 21-year-old female student.

Mr Kang admitted that his learning curve was steep, recalling that in his first few cases, his video clips of targets were shaky.

"When I first started, I would worry if the subject knew I was following him. I would get nervous whenever they looked in my general direction," he said.

Private investigators feel that their work could help clients to move on with their lives.

"Ultimately, we hope that clients can decide what to do next," said Mr Kang. "If couples can sit down and talk, maybe there is a chance that their marriage can be salvaged."