Julia, for me, it's the same from sketches as it is in the field--a combination of body shape and field marks. General impression and shape is very important in IDs. For instance, body profiles are very telling--look at the difference between the body shape of a nuthatch and a
chickadee. The nuthatch is weight-forward...the chickadee is chubby
with a more upright posture. General impression can also include certain behaviors in the field which, of course, the sketch can only hint at--perhaps a double-leg scratch during foraging as opposed to a single-leg scratch.
Shape is a biggie, too. Male Blue jays, Tufted titmice and Northern cardinals all have crests, but the male cardinal has a black mask surrounding the bill; the titmouse has a dark mark above the bill and between the eyes, and the bluejay has black along the top of the bill connecting to a thin eyeline that extends back to connect in turn to the black along the crest. The patterns are very distinctive. So a sketch with a crest and one of those patterns is easy to identify!
Hence my goal to get good/quick enough to capture the essence of an unknown bird in body shape/posture and still have time to note enough field marks to be able to make the ID later...
As for Fred's sketches, his dove has a classic dove shape to it and combined with the neck ring and the patterned undertail coverts, it becomes a Eurasian collared-dove! The titmouse has the crest, big eyes, and sort of a classic erect posture that screams 'titmouse'. Fred's later comment about the Black-crested titmouse from TX reflects on the pale face and lack of black spot between the eyes, field marks for that titmouse, instead of the Tufted titmouse we have here.