I don't know how to tell if it was feeding. Two of these were standing on the stem of an Ampelopsis brevipedunculata, but it was only a short hop from Rosa multiflora and Parthenocissus quinquefolia.

8 feet above is the lowest branch of a Tilia americana.

Not far away: a Cornus, a Fraxinus, Hedera helix, Celastrus orbiculatus, Pinus strobus, Lonicera japonica, Rubus phoenicolacius, Toxicodendron radicans, a Picea (abies?), and numerous non-woody plants.

(Obviously I'm newer to bugs than plants.)

Finally found a list online of host plants for Enchenopa (is there one on BugGuide? I couldn't find one here).

We do have 2 Cercis canadensis about 40 feet away, and it is possible that my Celastrus is actually scandens not orbiculatus (about 10 feet away). Then there are at least three varieties of Viburnum all 40 feet away.

Celastrus is more probable than Viburnum; the "horn" on the Celastrus-feeding species is more strongly curved at the tip.

The leafhopper is out of focus, but its relative size suggests a Ceratagallia.

BugGuide doesn't exactly have a list, but you can see what most of the host species are here, and you can see the host plants for the species for which it isn't obvious by clicking on their names and viewing the guide pages.

My understanding is that E. binotata is by far the most common species, and that it feeds on both species of Celastrus. I have found the egg masses on C. orbiculatus. Hopefully Dr. Hamilton will chime in...

I went back and checked; both are still on the Ampelopsis, not far from original spot. Can't find anything on my Celastrus.
How do you tell if they are feeding/have fed?

their beaks will be inserted in the stem. You would need better resolution to be able to tell from these photos. Feeding from insects with sucking mouthparts shows up as discolored spots on the plant, but on a pale brownish stem like this I don't know that you would be able to see any difference.

As I possess nothing fancier than a Kodak point-and-shoot (and couldn't even locate a hand lens) I went back to see if I could tell with my bare eyes if it was feeding. Found the same type of bug, this time under the shoot (its quite hot and sunny) and slowly got closer and closer. The front end was in contact with the stem but I could not see its beak (before it jumped onto my chest!).

I did, however notice tiny slits on the shoot, in a line, the last one exactly where the mouth had been. I looked at other Ampylopsis on the other side of my yard; no similar marks. Brought it inside to photograph. Each slit is almost exactly 1 mm. There are three evenly spaced ones on this side, and five others in a wobbly line backward a bit along the stem, back toward the darker upperside similar to where I saw one this morning. See 3rd image. What do you think?

These almost seem like they could be oviposition slits (where the treehopper is inserting eggs). Not sure what to make of that! Let's hope Andy Hamilton, our 'hopper' expert, sees your photos and comments.