Evaluating yearlings can be tricky as they may be at awkward stages of growth. However, I still look for proportionate body parts, correct angles and structural correctness: The legs, as viewed from front, side and rear, are straight with the forearms directly over the knees; the cannon bones set straight under the knees without deviation from plumb; and pasterns with the same angle as the hoof and shoulder. You can expect a yearling to look gangly and high in his hind end during growth spurts, but don't expect that he'll "grow out of" a short neck or straight shoulder.
See how I placed these yearlings in the January '09 issue of Practical Horseman.
Conformation Clinic appears monthly in Practical Horseman.