In essence, you do a separate takeoff of the same job and look for mistakes in the trainee's estimate. When you have finished your separate takeoff, sit down with the trainee, praise what he did right and explain what he did wrong. Always allow the trainee as much time as he needs to do an accurate takeoff. What might take you an hour could take the trainee half a day. What might take you a day might take him a week.
Speed comes with practice and experience.
Remember, a hurried estimator will always be stressed and a stressed estimator will always make mistakes. So be lenient and allow your trainee enough time to do accurate takeoffs. Patience works in the other direction, too. Estimating can sometimes be terribly tedious work, and patience with it isn't only a virtue, it's a necessity. Just as a painter who gets in too big of a hurry can slop paint all over the place, an impatient estimator can slop your money all over the place.