An excerpt from an item at the 24 Hour Museum:

Research has shown that Roman soldiers based at Hadrian’s Wall would have had a military tattoo, and the exhibition even explains the technique they would have used.

“It is a little-known fact, but it would appear that all of the legionaries and some of the auxiliaries on Hadrian’s Wall would have had a tattoo,” says the university’s Director of Archaeological Museums and expert on Roman history, Lindsay Allason-Jones.

Ancient symbols are often used in contemporary tattoos. Photo Newcastle University
photo of an arm with a knotted cross tattoo design on it

Evidence for the practice comes from the Epitome of Military Science, written around the 4th century AD by the Roman chronicler Vegetius. He recounted that recruits to the legions would have to earn their tattoo once they had been tested by physical exercises.

“We do not know what this official mark looked like,” says Lindsay. “It was possibly an eagle or the symbol of the soldier’s legion or unit.”

The 6th century Roman doctor Aetius recorded that soldiers sported tattoos on their hands and detailed the method they used to create them, noting how leek juice was used as an antiseptic to wash the area to be tattooed.

Designs were pricked into the skin with pointed needles until blood was drawn before the ink was rubbed on, which was made of Egyptian pinewood, corroded bronze, gall (bile) and vitriol (sulphuric acid), plus more leek juice.

... you have to visit the site and see the photo of the tattoo on one of the visitors to the exhibit.