Thanks to all you helpful responders! Your ideas included: the book "Peterson Guide to Bird Identification -- in 12 Steps" by Howells and Sullivan; the Merlin app; writing down notes and later hitting the field guides [my process exactly!]; Cornell's "All about Birds" site that actually gives you access to Merlin; the subreddit "What's That Bird?"; and the forum WhatBird.
Well, folks, we have an apparent winner, or at least I feel I can go ahead and complete my eBird list. Sunday evening I stopped briefly at BT Anderson Park in Richmond, where a Robin-sized bird was taking turns with Robins, flying in and out of the same bare treetop at the bridge. It appeared to be sallying like a flycatcher. It looked gray and fairly featureless to me, more slender than a Robin, light-colored bill, no eye-ring, barely perceptible wingbar (or bars?). My first thought was Solitaire, but -- no eye-ring. Then female Oriole of some sort -- but the bill was wrong, just not a decurved icterid bill. Then it came to me -- Ash-throated Flycatcher -- but no, it was sallying upward, not to the ground, and this bill was too light, and, and. How about Greater Peewee? Female Summer Tanager? Bill too thick. Western? Well, yes. The last photo on Merlin looks about right.
Here's a link to my eBird entry so you can see the photos and my comments: https://ebird.org/checklist/S74198790
Oh, and let's try not to remember how notoriously quiet and inconspicuous Tanagers actually are, so could somebody please find a reference to female Tanagers flycatching from treetops a half-hour before sundown? ?