You could look through the job profiles on the Learn Direct website. These indicate the qualities and qualifications which are required for each type of job:
http://www.learndirect-advice.co.uk/helpwithyo urcareer/jobprofiles/
That assumes that you're approaching the problem from the point of view of the young person (or their parent).
If you're an employer trying to select between several candidates, the first thing to do is to list the qualities that
really matter for a particular career. For example, lots of employers (wrongly) look for things like a good GCSE result at mathematics when they know full well that the employee will never encounter a sine or cosine in the job. A good employer would put things like 'ability to relate well to others' ahead of simple exam results. The obvious way to assess such things (other, of course, than a simple interview) is to offer a short work trial.
Chris