Ajit Singh, a “wedding detective,” scans a paper Dec. 15, 2009, at his office in New Delhi.

New Delhi ? Ajit Singh knows about the lies people tell.

He has followed them through the littered, mildewed mazes of New Delhi’s middle class neighborhoods. He has photographed them as they leave their lovers’ apartments. He hears them exaggerate their salaries and hide their illnesses.

A thin man in an ill-fitting suit, Singh works out of a crowded office around the corner from a muffler shop. An incense stick burns behind his desk. A sign in slightly fractured English warns the staff: “Walls Has Ears And Eyes Too. BE ALERT.”

Singh has spent years honing his skills: disguise, surveillance, misdirection. With just a few minutes’ notice, he can deploy teams nearly anywhere across the country.

Because in modern India, where centuries of arranged marriages are being replaced by unions based on love, emotion and anonymous Internet introductions, where would a wedding be without a private detective?

“Today, there’s a need to check if people are telling the truth. And that is where we get involved,” Singh said. “Does that boy really have an education? Is he really earning that big salary? Is that boy or girl running around?”

A groom-to-be may seem like a nice young man. He might come from a good family. But nearly two decades running his own agency, Hatfield Detectives, has taught Singh how little that can mean. So he spells out a warning, lingering over each word: “You don’t know what that boy is doing with his time.”

The detectives, though, are ready to find out.

“Before, we were only a luxury for someone who had a hell of a lot of money,” said Baldev Puri, the tough-talking 45-year-old founder of AMX, a large New Delhi-based agency where one-third of the business is premarital investigations. “Now, every family wants to know the maximum.”

Want to have your daughter’s fiance followed? Want to know his salary? Does he drink? Smoke? Hang out in bars? What’s his blood type?

No problem. It’ll cost just $300 or so for a basic investigation (surveillance teams are extra). India has a per capita income of less than $900 per year, but detective bills running into thousands of dollars are increasingly common.

The arranged marriage was long the ironclad norm in India: Two sets of parents, aided by matchmakers and older relatives, would choose spouses for their children. Trusted friends were consulted to look for signs of trouble: a potential groom about to lose his job, or a potential bride too flirtatious with the neighbors. The couple-to-be were, by and large, left out of the discussions.

But as views on relationships shift, and an ever-growing middle class grows desperate to ensure that their children’s marriages don’t end in disaster, a small army of detective agencies has sprung up across the country. A profession that once lurked on the shadowy fringes of society has moved quietly into the mainstream.

These days, Western-style “love marriages” — with all the messiness that comes with love — are increasingly common, particularly in larger cities. Even the more conservative families are increasingly turning to “arranged-love marriage,” where parents set up potential partners but leave the final decision to the couple.

Not surprisingly, the detectives are a fairly hard-bitten lot. They swear often. They’re not easily given to trust. They suspect most people are hiding something. Secrecy is exceedingly important to them. To be photographed for this report, Singh wore sunglasses and a wide-brimmed hat to hide his identity.

Many detectives see themselves as a combination of helpful uncle and Oprah Winfrey. They love telling you about the relationships they have put right — even if it means putting those relationships asunder.

There was the groom-to-be who turned out to be HIV-positive, and the woman using marriage Web sites to troll for gift-bearing boyfriends.

There was the woman from the well-to-do family who called Singh when her fiance began switching off his cell phone for a couple of hours a day. The marriage was just two months away and it made her family nervous. Still, she insisted to Singh, he was a good man: doesn’t drink, doesn’t smoke, lives quietly.

But the surveillance team found something different.

“We found him roaming around with his ex-classmates. He used to go into restaurants, smoking and drinking,” Singh said. Sometimes there was a girlfriend roaming beside him.

The wedding was quickly canceled.