Don't worry about copy variation unless you can visually see something wrong with either the lens or the product. For the most part, slight variation in quality is undetectable, and unimportant regardless.
Originally posted by DanWeso
How does a fairly new dSLR user know if they have a good copy of a lens when you don't really have anything to compare with it?
They don't really. If you buy the lens new, you can exchange and exchange until you are happy... but there is no assurance that you will end up with a better product.
I would suggest you don't even think about it unless the pictures you are getting make you unhappy. Sounds to me like you are satisfied with the sigma and the kit lens, which make a good pair.
I think it is better to think but lenses in their utility, rather than in IQ alone, because it is a subjective measure for most users, where MTF charts are very theoretical in their application to a photo.