Alitaptap,
A relatively simple way to meet this requirement would be to have some kind of record of assessment of relevant contracted employees - you could keep this very brief. It often works well to put it on/with the recruitment documentation, eg, an assessment after they start working for you by their supervisor/team leader/manager or similar.
I imagine this does happen at the moment but that it's informal and not recorded. All you need to do is build something into your system, and have at least one record that it was done. Naturally, if they're not competent for the work you need to do something - whether that be training & further assessment, or replacement.