Hi Nevada birders,
Here are a few things to look for on potential first-cycle Slaty-backed Gulls, specifically in comparison with Glaucous-winged x Herring hybrids:
1) Short wings. This is often used by the Japanese experts. Note that this species seems to have exceptionally flexible wings, often extending the humerus. In combination with folding the secondaries inward, this can give the wings an unexpectedly long, narrow look.
2) More specifically, short primaries. The length of the 10th primary is often notably shorter than the distance between P10's base and the bend in a partly spread wing. Glaucous-winged and Herring have longer primaries that often exceed this distance.
3) Short, stout bill with minimal gonydeal angle.
4) Contrastingly dark auriculars.
5) Plain-based greater coverts (dark brown when fresh, white later). This and all similar species have extremely variable greater covert patterning, however. Note that Thayer's does consistently seem to have white barring and mottling toward the bases of these coverts.
6) Distinctly down-curved gape (shared with California but generally not by Glaucous-winged or Herring).
7) Long mid-secondaries. These look like a point or bulge when spread, much like a skua. This is not obvious when they are more folded.
8) Contrastingly white background to rump. Many Slaty-backed Gulls have dark rumps, especially when fresh. A contrastingly white background, however, is also common and would differ from both Glaucous-winged and Herring.
9) Many birds have a rather blocky-looking head shape, with a concave slope to the forehead.
10) On many birds, the inner primaries are darker than either Glaucous-winged or Herring. The "shadow string of pearls" effect can be quite striking.
11) When resting, Slaty-backed exhibits a very forward-hunched, chest-heavy posture. The legs vary greatly in length but are often notably widely-spaced, and the gait has been described as waddling or goose-like. When alert, the neck looks very long and the head rather small.
With those tips, I'll let you all decide whether the Sparks Marina bird is a solid "pure" Slaty-backed. There are a few things (fine speckling on upperparts, slightly long primary projection, general smudginess) that might represent the influence of some Glaucous-winged genes, and these species are known to hybridize. These characters can be found on birds commonly called Slaty-backed Gulls by Japanese experts, however.
I have discussed some of these points in more details in the comments on the photos of the Arcata bird:
Best of luck!