All 8 characteristics used to determine whether "pure" Hermit Warbler or hybrid with Townsend's Warbler indicate "pure" Hermit Warbler:

Mid flank -- no streaks
Lower flank -- no streaks
Bib corner -- gap between bib & back
Face -- no black in face (part of branch in foreground shows over bird's eye)
Extent of yellow -- no (0 mm) yellow below bib
Intensity of yellow -- no yellow at edge of bib
Crown -- most yellow on crown
Back color -- gray, no evidence of green

Also, the undertail coverts were "immaculate" as in Hermits & not in Townsend's, a character that wasn't used for the study because they determined it to be accurate but "not worth the effort."

From article in the Auk, Vol. 115 No. 2 April 1998, pp. 284 - 310, published by the American Ornithologists' Union.
Three Hybrid Zones Between Hermit and Townsend's Warblers in Washington and Oregon, by Sievert Rohwer and Christopher Wood.

Available as an Adobe Acrobat document (.pdf) from:
Three Hybrid Zones...

Thanks to Andy Stepniewski for lending me his photocopy of the article & describing where he found a Hermit Warbler about a mile from where I found this. There was also a Hermit/Townsend's hybrid in the area that this male chased off several times.

Later I found 3 Hermit Warblers together about 2 miles from here, an adult male & 2 immature birds. I also found 3 other Hermit/Townsend's hybrids in the area.