If you were supervisor for a firm that assembled car parts and parts are always being returned to a specific group to “do over” how would you go about assessing whether this is a training factor?
If your employees are doing a bad job, there are three major factors that could potentially be at fault. First, there is the issue of their qualifications. Second, there is their training. Third, there is the issue of how they are lead and managed. What you need to do is to find a way to determine the degree to which training, rather than one of the other factors, is at fault.
To do this, you need to isolate the effects of training. The question implies that there are other teams performing this same work. You need to compare the underperforming team with the other teams. First, you look to see if they have the same qualifications. If your bad team has less qualified members, that may be your problem. Then, you need to look at leadership. Are the teams all managed by the same manager? If not, you may need to look at the quality of the management that supervises the team. If the bad team has the same level of qualifications and is supervised by the same managers, then there may be an issue with their training.
In other words, what you have to do here is to try to make sure that the bad team is like the good teams in every way other than training. If you can establish that, then training must be the problem. If there are other differences, it will be extremely hard to determine the degree to which training is the problem.