Another approach that many experienced managers use is to help their new reports create a 30/60/90-day plan. This can include basic goals, like getting up to speed on the code, committing a bug fix, or performing a release, and is especially valuable for new hires and people transferring from other areas of the company. The more senior the hire, the more he should participate in creating this plan. You want him to have some clear goals that will show whether he’s learning the right things as he gets up to speed. These goals will also require some work from you and from the team, because it’s very rare that everything is self-evident, well documented, and totally obvious to a newcomer.
Unfortunately, sometimes you will mishire a person. Having a clear set of expected goals for your new hires that you believe is achievable in the first 90 days will help you catch mishires quickly, and make it clear to you and to them that you need to correct the situation. Create a set of realistic milestones based on your prior hires, the current state of your technology and project, and the level of the person coming in.