Frescoes tend to be a decent source...
There has been some discussion about how much stylizing gets put into them, but some prominent historians (Dr Tim Dawson) think that the stylizing though prominent, does not include equipment that was not used...
So basically... they are just really pretty versions of the actual gear...
Though there are some conventions, such as the legs... usually the lower portions of the figures are done in a classical style, such as sandals and the like, which were hardly used by men-at-arms on campaign, but barring that, what was depicted is what was common enough to warrant depiction.