The Harrogate Autumn Flower Show will use specialist technology to ensure the winning fruit is of the Gigantomo variety.
The giant entries must be from that strain of plant to win the Â£1,000 prize.
It comes after competitors raised concerns unscrupulous growers might sneak in a rogue type of tomato.
Show director Nick Smith said they wanted to be certain the winner "had the right pedigree."
"Giant veg growing is great fun and tremendously popular with our visitors, but it also has a serious side, especially with such a big prize at stake," he added.
The Gigantomo class, developed for its huge red fruits, was launched in January, with the final weigh-in on Friday.
Plant specialist and prize sponsors Van Meuwen said it is the product of almost two decades work by breeders in the UK and United States.
Dutch specialists will test the plants to make sure the entries are what they claim to be and the winner could scoop a further Â£5,000 if the heaviest fruit sets a new world record.
The Guinness record for the heaviest tomato stands at 7lb 12oz (3.5kg).

The woman was picked up from Candlemaker Row, Edinburgh, near the Greyfriars Bobby's statue, at 03:30am.
She was taken to a layby just past the Hillend Ski Slope, near the Boghall Farm in Midlothian where she was subjected to a serious sexual assault..
She was then left near the Sheriffhall roundabout on the city bypass.
Police Scotland are appealing for any witnesses who may have seen the victim or the car.
The suspect was described as being of South Asian appearance, aged 35-40,  about 5ft 5in - 5ft 6in, of slim build.
He had dark, cropped hair and was clean-shaven. He was wearing dark clothing and spoke with an Edinburgh accent.
The car was described as being a five-door saloon, which was possibly a navy colour.
Det Sgt Keith Mackay said:  "This was a frightening experience for the woman, who is now in the care of her family.
"We are eager to trace this man and are appealing for witnesses.  Anyone who recognises the man's description, or the description of the car, is urged to come forward."
Gardeners trying to cheat their way to the top tomato prize at a horticultural show will be weeded out with DNA tests.
[NEXT_CONCEPT]
A 31-year-old woman has been sexually assaulted after getting into a car she believed was a private hire taxi.