If you're trying to hire developers who have open source experience, you have a big advantage compared to hiring other kinds of developers. Most of the résumé of an open source developer is public — it's everything they've ever done in every open source project they've ever worked on, because all of that activity is publicly archived. But you shouldn't need to go searching for all of it. When you put out a job posting, tell prospective candidates directly that the résumé they send in should include references to their open source profile. This means their committer accounts on the projects where they've been active (or their account names at the overall project hosting sites where they're been active, e.g., their GitHub.com username), the email addresses or usernames they have used when posting in discussion forums, documentation they have written, and anything else that would lead you to places where you can see their open source project activity.
The kind of activity to look for is not only their direct technical activity, but also their relations with the other developers in the project. So look at the candidate's commits, but look also at the frequency with which they reviewed others' commits, and at their reaction to reviews of their own commits. In the project's issue tracker, how often did the candidate respond constructively to incoming bug reports or contribute useful information to a bug ticket? Visit a threaded view of the project's discussion forums and see how often the candidate's posts were responded to, and what the general tone of the responses was. Someone who consistently causes negative reactions from others in the project may have social problems as a collaborator, which is important to know independently of the candidate's raw technical ability.
If candidate is applying for a position that would involve working on an open source project, but seems to have little or no open source experience themselves, this is not necessarily a showstopper, but it's a sign that you should ask some probing questions, and that you should expect some ramp-up time if you hire them. If the candidate is young and inexperienced in general, then lack of participation in open source is easy to understand. However, if the candidate has been a programmer for a while, and especially if they already have experience as a user of some of the open source software you'd be hiring them to work on, and yet they have never participated much in that project except to download and use it, then you should ask them questions about why. There is nothing wrong with being uninvolved as a participant in software that one uses. However, if you're hiring someone to be a participant in a project, and they already had a chance to be and chose not to, that could imply a lack of intrinsic motivation to participate and may indicate that this person's temperament is not what you're looking for. Or there could be other reasons — for example, the candidate's prior management forbade them from participating. Whatever the reasons are, you should make sure you find out.
It is very common for companies to hire an open source developer precisely because of her existing position in an open source project. She may be the founder or leader of the project, or may just have commit access, but either way her ability to get things done in the upstream community is part of her value as a prospective employee; often, it is just as important as raw technical skill.
As noted in the section called “The Economics of Open Source”, there is nothing wrong with purchasing influence in this way, as long as the employer understands that the new employee will have dual loyalty. It is inappropriate to ask the employee to take actions that would harm her standing in the project. The employee's manager needs to be sensitive to this, and to let the employee know that the door is open for discussion and pushback if she ever feels she's being put into such a situation (hence the importance of managers who understand open source, as described in the section called “The Key Role of Middle Management”). It is perfectly fine for the employee to promote the company's technical interests in the project, and to do so openly, as long as the proposals are compatible with the project's overall goals and the company provides resources to support those proposals in a way that's sustainable for the project.
Remember that influence in an upstream project is usually not transferable to some other employee. Position and influence travel with the person, not with the employer. There are occasional exceptions to this, e.g., in corporate-driven projects where the balance of power among competitors is especially important, or in standards bodies with formal representation policies. In these cases, a governance committee seat may be reserved for a certain company, and the company gets to designate who sits in that seat. But even then, informal influence tends to matter a lot, and individuals may not be truly interchangeable in practice.
This makes the recommendations in the section called “Hire for the Long Term” all the more important. When an employee holds a position of influence in an open source project that is strategically important to your company, that employee has a pretty good bargaining position.
Since that kind of employee is likely to be with you for the long term, try to take advantage of it by having her help onboard others into open source projects. Nithya Ruff, Director of Open Source Strategy at Western Digital, told me that when her company acquired another company that had a history of working on certain strategically important (to the acquirer) open source projects, the engineering team that came with the acquisition became a strong influence inside the newly combined company. The developers had good reputations in the upstream projects, and the new management not only made sure they were able to continue working in those projects, but brought them into a company-wide open source working group to help other engineers get involved in upstream maintenance too.
 Brian Fitzpatrick has written about the usefulness of having an open source résumé in two articles, The Virtual Referral (http://www.onlamp.com/pub/a/onlamp/2005/07/14/osdevelopers.html) and The Virtual Internship (http://www.onlamp.com/pub/a/onlamp/2005/08/01/opensourcedevelopers.html).