The centurion's armour varied widely, perhaps it was even a matter of individual choice. The centurion depicted wears leather. The rings (torques) and circular plaques (phalerae) on his torso are awards he has won in battle, comparable to medals in modern day armies.
The horsehair crest across his helmet made him easily recognizable by everyone, even in the chaos of batte, as a officer of considerable rank.
The greaves he wears to protect his shins are also a sign of his being a centurion. A cape was usually also seen as a mark of a centurion. Although, given that their style of dress may well have been down to their own choice, this centurion may not be at all out of place without such a cape.
Note also the vine rod he holds in his hand. A further insignia of his rank and one he would happily use to beat unruly soldiers with to enforce discipline.