Darley's No.1 stable jockey Kerrin McEvoy will take over on Caulfield Guineas favourite Long John when the gelding faces his biggest test on Sunday.

Long John has won four of his five starts with Steven Arnold aboard, including an impressive 2-1/2-length victory second-up in the Listed Henry Bucks Stakes at Flemington earlier this month.

McEvoy will be in Melbourne for Sunday's Caulfield meeting and will take the ride in the Guineas Prelude (1400m).

Long John is $4 favourite for next month's Group One Caulfield Guineas (1600m).

Trainer Peter Snowden said Sunday's race would provide a better guide to the son of Street Cry's Guineas chances.

"It will be a good test for him. He'll meet some better class horses but you can't fault his form," Snowden said.

"I've probably under-rated him a little bit. For me he works nicely but he doesn't work like a Group horse. He certainly races like one."

Snowden said Arnold had given him a good report on the gelding and the trainer was also glad to see the Gai Waterhouse-trained Rockford win at Caulfield on Saturday after finishing fifth to Long John in the Henry Bucks.

"The bar gets higher and higher over the next couple of weeks so we'll see what he does on Sunday," Snowden said.

Snowden, meanwhile, was delighted with dual Group One winner Epaulette's work at Moonee Valley on Monday ahead of the horse's return in Friday night's Group One Moir Stakes (1200m).

Having his first look at the tricky Moonee Valley circuit, Epaulette worked with stablemate Chiaramonte and was slightly in front as the pair crossed the line.

Epaulette was second to Black Caviar in the TJ Smith Stakes (1200m) first-up last preparation and Snowden said his fresh form was "impeccable".

"All I wanted to see was him get around the corner well this morning and he was on his right leg and he quickened nicely off the corner," Snowden said.

"I know it's going to be a good field on Friday night but the horse is going very, very well."