I am an engineer. But have done a lot of hiring, both technical and non-technical. I now run
http://InterviewKickstart.com.
The core thing you have to realize, is that there is NO reason for a non-technical person to not be as smart/driven as an engineer. I am not saying you believe that, but what I'm saying, is that while the questions may be different, your grading standards should not change. Especially not when you're a young company growing fast. The hire you make now, will likely be with you for a long time.
Having said that, the following have worked well for me in the past:
1. Ask them simple logical brain-teasers. The kind that don't need programming knowledge. You are looking for excitement and a drive to solve them, instead of backing down.
2. Ask them to present something to a small group of people. It could be their favorite project, or their hobby or whatever. You are looking for clarity of thought, the ability to explain something to a few people, the depth of interest, empathy for the audience and confidence in themselves.
3. Ask them for a writing sample. This is very much akin to a code sample. Is it clear to understand? Is it engaging? Have they taken care of details and punctuation? Very important.
4. Put a few different personalities in the interviewing panel. Are they able to hold a conversation with everyone? Are they able to treat them with respect, regardless of the personality type?
5. Ask them what they want to do in short and long term. Is there clarity? Is there ambition? Is there reason and grounding?
Good luck!