As with Prime Movers and End Users, bring a list of participant-appropriate topics (see item above) and probes with you to each interview, upgrade your list as participants ask things you hadn't anticipated, take note of their answers, start with open probes and close each topic with one or more closed probes and do everything you can to actually LISTEN to what they tell you.
Look for tell tale signs that suggest the participants' level of organization and time management ability. These signs include file/office orderliness (pile file system vs. actually using the cabinet), ability to retrieve information for you quickly and the use of a non-computer-bound planner. (People whose only planning tool is an application on their desktop computer can easily get overbooked when project work takes them further than the cables/cords will allow the computer).
When probing for signs of project-specific competence, focus on their actual and relevant work experience rather than asking general questions about their ability. (Few people willingly admit to ignorance, but most people will honestly tell you what they have and have not done.)
Probe for signs of project enthusiasm (you're trying to gauge the priority that they'll give the project and just how much 'selling' you'd have to do to get enough of their time). Rather than asking "Is this project something you think you could really support?", ask a probe that doesn't signal the 'right' answer so clearly; eg. "If you were to sign on to this project, what do you think would be the most interesting (challenging, stimulating, satisfying, etc.) aspect of your work?"