William,
I'll hope you will forgive me for simplifying things a lot, as compressing years of lectures, reading and research into a forum post is a bit challenging.
As with written sources I have to evaluate them by asking a lot of critical questions and trying to answer them:
What knowledge did the painter have of of the subject of the painting and the specific items he painted in it?
Here artists who were also veteran soldiers get a high rating, Paul Dolnstein, Urs Graf and Nikolause Manuel Deutsch all had extensive experience with soldiers clothing, arms and armour and it shows in their drawings. As a result they get a high rating.
Jan Cornelisz Vermeyen was not a soldier himself but accompanied Charles V on the Tunis campaign with an order to sketch the events, people and landscapes in order to create a first hand record of the campaign for posterity. So he also get a high rating.
Frans Hogenberg did not have any first hand experience with the Danish-Swedish war of 1563-1570, nor did he have much in the way of information from secondary sources. As a result his images of that war gets a low rating.
Why was the image made and for whom?
Was the image made to be a realistic portrayal or is a part of a religious painting, an allegory or a satirical piece?
For example:
I was once in a discussion where this image
was used as proof that peasant were used to fill out the rear ranks of pike blocks. The not so small problem is that it a cropped version of a much larger print, a look at the intact image shows that the landsknechts are fighting an army of priests, monks and devils(!) led by the Pope and that the entire image is a moral-political propaganda piece connected with the reformation rather than an attempt to show an actual battle formation in a realistic manner.
Urs Graf made several satirical drawings of Landsknechts which exaggerate certain parts of their apperance and equipment. This picture is one of them
Is there supporting sources and evidence?
You compare the content with other sources and evidence, i.e if a an eyewitness describes French ordonnance Archers in 1494-95 as being armed with longbows and wearing breastplates then an engraving of french troops with that kind of equipment is probably correct in that part.
On the other hand there are a number of prints showing Ottoman troops fighting in deep pike squares, something which is not at all supported by other sources.
Has artistic license been used?
A good example of artistic license to watch out for is the habit of artists to depict historical events using contemporary soldiers and clothing. So you get Albrecht Altdorfer painting Alexander the Great riding into battle in golden full plate armour while the Macedonian phalanx is a landsknecht gevierthaufen and so on.
Another example is Pieter Snayers who was_the_battle painter of the TYW period, in many of his paintings he depicted his infantry units with more pikes and in deeper formations that were actually used at the time. Mostl likely because they looked more impressive that way in his opinion.
What are the flaws & limitations of the image or the artists style? Why are these "errors" present?
Are details missing or wrong, and why are they missing or wrong? For example with a few exceptions Paul Dolnstein did not draw men wearing swords in his drawings of larger battles, probably because doing so would have been difficult and cluttered up the image.
A painting's value as a source may be limited, for example it may not be a usefull representation of tactics or tactical formations but could still give very usefull information about the dress and equipment of the soldiers. The 1502 painting is a good example of this.
I hope this made the methods I use a bit clearer?
Extant clothing is a very rare thing from the 16th Century, we mainly have to rely on pictorial evidence combined with written sources. In some lucky cases the "pattern books" used by tailors have also survived. Arms & armour is easier as we have a lot more surviving pieces to compare with not to mention the written sources that provide information about armour such as eyewitness accounts, muster rolls, militia ordinances, equipment purchases orders and so on.
Experimental archeology and reconstructed clothing/gear also plays a part as it helps you judge if a garment or piece of armour is plausible or even possible.
Daniel