One of the most tell-tales signs I would say is itchiness (and hence scratching), especially the nose and chin area. IIRC, codeine in particular can also make your upper back, shoulders, and neck really itchy. In men, opiates often make your privates pretty itchy, too...in particular, the 'lower parts', shall we say. Satisfying this itch requires a bit more 'action' than your typical 'male readjustment' manuever, too.
Watching for a lot of scratching in these regions, along with unusual 'animation', talkativeness, and pinpoint pupils should give you a good idea if someone is high on opiates. As ExJ said, lower 'keeping from getting sick' dosages in the case of an addict won't produce much in the way of noticeable signs.