Once you have completed an initial screening, behavioral style interviews, and skills testing, you should have a shortlist in single figures or ideally, even a clear favorite that you want to hire.
I would recommend that the next step would be a reference check, especially if the candidates have been employed before. You could also get a character reference from a sports coach or teacher to get a day-to-day insight into the person. The purpose of a reference check is to look for confirmation of the skills and attitude that you have witnessed in the interview process. Most people will give you a positive reflection, but when it is decidedly neutral or negative, you may want to investigate your candidate somewhat further.