A case in which argument has focused on whether a trainer was wearing underpants when stewards raided a Melbourne racing stable is set to test the credibility of Racing Victoria's Compliance Assurance stewards.

The stewards allege Adelaide trainer Paul Beshara injected the horse Happy Trails with an unknown substance on the morning before it was due to race at Moonee Valley last month.

As well as a failure to locate any syringe or needle, samples taken from the horse revealed no trace of drugs or any other substance.

But under rules that prohibit any form of treatment on race day, Beshara faces charges that could cost him at least a six month disqualification.

The Racing Appeals and Disciplinary (RAD) Board on Monday heard stewards first inspected Happy Trails at the Caulfield stables at which he was staying early on the morning of September 14 and found no sign of any treatment.

They again examined the horse at 11am after Beshara had arrived to tend to the horse and noticed a wet patch on its neck. A third examination six minutes later revealed a haematoma and a puncture mark in the vicinity of the wet patch.

The stewards therefore concluded Beshara had injected the horse.

The RAD Board heard evidence from a foreman at the stable that the stewards' initial examination had been inadequate and had only involved running an electronic scanner over the horse to identify it.

Dan Williams, the foreman at the stables housing Happy Trails, agreed with a suggestion that the stewards had lied and that their version of events was a "concoction".

But counsel for the stewards, Dr Cliff Pannam, submitted that Williams evidence "taxed credulity".

"This is a dramatic series of allegations against (the stewards)," Dr Pannam said.

"It is simply not credible to accept his evidence."

The Board heard Beshara had complied with a request from stewards Dion Villella and Kane Ashby to turn out his pockets, but he had refused a request to drop his pants.

Beshara said he hadn't been wearing underpants and had been reluctant to remove his jeans when requested.

He also suggested the absence of his underwear supported his assertion that he hadn't hidden any hypodermic needle in his pants as suggested by the stewards.

The RAD Board will hand down its decision on Friday.