How to gauge your own work?

[topliner]

I think you answered your own question.

Since you can't really quantify art in the same way that you could for, say math (ie being at a grade 9 level versus grade 4 level, for instance) the only way is to compare to other artists.

That being said, its important to know that nobody develops at the same rate, and also that there isn't really a way to measure up to other artists. For example, a lot of Dave Rapoza's work is super detailed and well rendered, but does that make him "better" than someone whose work is more stylized, say someone who does art for kids movies? Or say a company like Riot where a lot of the art is more cartoony?

Not really. Its just different. There is of course a certain amount of technical proficiency that you should strive for, but generally speaking that falls under the realm of clarity rather than ultra detail