One way - at least one i use with students is something like this - said nicely and genuinely. Take the person back to there task and try one of the following, "Does this look done to you?" Even kindergartners respond well and will take that look. If he says no - ask him to point out what needs improvement, then ask him to finish up. if he says yes, point out what he's missing. I know it seems like you shouldn't have to, but even in the upper grades I'll use this tactic for a missing name - which happens all the time. it teaches them to double check and forces them to learn. :)