two things you should consider (after looking into the slides they present):
- don't hire someone who leaves before the suggestions are implemented. so each project tries to involve external consults should make sure that the involved people are not only responsible to provide some kind of "best practices" but also to implement their usage in the particular case they are hired
- don't hire someone who do not eat his own dog food. means ask you're potential consultant how comparable cases are solved by his own company or his or her own daily business.
each problem can be traced down to some analogue problem which requires the same approaches you faced with. if you talk about the daily work and solution with your consultant before hire him you get a good feeling if the external person really knows what he talking about