Feral Parrot Populations in San Francisco
The main things to look for on perched birds are:
(1) presence of white or yellow in the secondaries: this is diagnostic for White-winged; however, the white on White-winged is sometimes (rarely) not visible on the folded wing, so its apparent absence does not confirm a bird as Yellow-chevroned. In Yellow-chevroned the yellow is limited to the coverts; on White-winged, even when no white is visible, the yellow extends back from the coverts to include the visible outer webs of some secondaries, giving the yellow wing patch a different shape.
(2) overall plumage color: much brighter yellow-green in Yellow-chevroned, as opposed to a darker, forest green in White-winged.
(3) bare facial patterning: both species have evident blue-gray bare orbital rings, but in White-winged this is expanded anteriorly into the loral area.
(4) under side of tail: strongly washed with turquoise blue in White-winged; plain green in nominate chiriri Yellow-chevroned (but beware possibility of behni Yellow-chevroned, which are bluer on the undertail).
(5) bill color: as pointed out to me by Donald Brightsmith of Duke University (who is writing the BNA account for these taxa), the bill of Yellow-chevroned is a deeper darker flesh-pink color than that of White-winged.
There are several published photos of Yellow-chevroned Parakeets, including in the Audubon Master Guide to Birding and in Birding 25:426; some of the plumage characters I mentioned are shown in a photo of specimens on p. 430 in that same issue of Birding.
Much more excellent information is available at the Parrot Project site.