What distinguishes this one from the Eristalinae? That is, what should you look for to tell that this is Syrphinae?

Go to the information pages for the two subfamilies: Syrphinae and Eristalinae and you'll find the answer.

The distinction appears to be whether or not the postpronotal lobes/humeri are bare or hairy (pilose). Is that correct?

Next question: what are the postpronotal lobes and where does one see them in this picture? I think these are the upper parts of the first pair of legs (i.e. those nearest the insect's head). Hairy thighs means this is Eristalinae. Bare thighs makes it Syrphinae. Is that correct?

Assuming that's all correct, given the resolution of the photograph, I'm not sure one can say with certainty that these legs aren't hairy. How obvious are the hairy legs in the field and in photographs of this resolution?

It loks like the info page needs some rewriting, including a link to the Glossary. Even that doesn't help much in this case.
I, actuallly tell the difference between the two subfamilies in most cases just by the general look of them. They are very different once you are used to them. Eristalinae are larger and also more stout. The pattern of the abdomen may look superficially similar to that of Eristalis but if you compare this one with Syrphus and Eupeodes you will see the similarity and you will begin to see the difference with Eristalis.
Sorry that I can'at be of more help.

Thanks for your comment. I'm amazed at how long these things hang on into the fall.