Perhaps it was because I'd just seen the trailer for the new James Bond movie, but I was hoping for trench coats or disguises when I sat down to lunch with a group of secret Thai-food tasters. Perched at one of the broad, high tables at Jarun Pompanya's Capital Thai, in Sydney, I was with three of the people best placed in Australia to confirm, in a Royal-Thai-Government-Approved kind of way, that what I was consuming was some of the best Thai food I had ever eaten and possibly would ever eat.

Under the watchful gaze of an enormous herd of elephants, carved frolicking in a massive slab of wood, I was dining with Saowanit Pongsai, the director of the Thai Trade Centre for Australia and New Zealand; Kwanapa Phivnil, her deputy; and Silachai Surai, the director of the Tourism Authority of Thailand for Australia and New Zealand - all of them seasoned undercover operatives in an ongoing and official covert mission to identify, ratify and promote authentic Thai food outside Thailand. I was close to the heart of what I'd come to think of as Operation Tom Yum - the lack of trench coats and disguises perhaps explained by this meal's status as an off-duty dining experience.

For more than two years, the Thai government has been sending designated diners into Thai restaurants around the world to sample menus, evaluate decor, assess preparation, cooking and presentation, and rate hospitality and service. And it's these secret diners who award - or withhold - the Thai Select tick of approval, Thailand's highest endorsement of any eating establishment. So far, judges have eaten in countries including China, Bangladesh, America, Brazil, Egypt, Turkey, France and Luxembourg, anointing more than 1000 Thai Select restaurants worldwide. Of these, about 50 are in Australia and New Zealand, including Capital Thai, which gained its tick of approval more than a year ago.

Under the system, restaurants that think they meet the Thai government's criteria can apply for consideration, after which a table of three or four people will appear unannounced - perhaps from the government's tourism or trade entities; perhaps from the Thai consulate or from Thai Airways - and place their orders. "We have specific dishes that we use," Saowanit Pongsai says, "like tom yum goong, the clear soup, and pad thai, the noodles - something that every restaurant should have, so we know that they're cooking it the way it's supposed to be." Rather than undergoing specific training, judges are required to have an innate familiarity with these standard dishes.

"Most of the judges are Thai, so they know exactly how the pad thai should be," Kwanapa Phivnil says - "you know, it shouldn't be too wet, too dry ..." Too sweet? The quantity of palm sugar in some Australian versions often makes noodles glisten. "In fact, no," Silachai Surai says. "But Australians do love sweet - when we try many Thai dishes here, they're often much sweeter than in Thailand. I don't know why this is. I've asked some chefs, and they say the customers love that taste." Maybe, he suggests, people think it will it reduce some spiciness. He shrugs: at least Australians are getting better at eating hot food - "spicy, but not too hot: that's how it should be."

Surai's time here as director of the tourism authority has corresponded with a 20% growth in Australian visits to Thailand; such holidays, he suspects, feed the world's growing penchant for Thai food. In excess of 13,000 restaurants worldwide now designate themselves as Thai, and Phivnil quotes research that rates the cuisine as the third-most popular globally, behind Chinese and French. "In India, Thai restaurants are becoming popular, and in Tokyo and other parts of Asia," she says. "If you go to Thailand and experience the food, you come home and look for it." Which is only logical: "After all, if you had to live 365 days in a year without any spice, that would be terrible ..."