What questions to ask prospect DBAs?

At my place of work we are going to be hiring a DBA to help us with a new system we are going to make. The problem is that we don't have anyone in-house with DBA experience so are unsure on what questions to ask on the interviews. Could anyone recommend some questions or general approach? We are using a few agencies and we are hoping they screen their candidates, but we still need some specific type of questions to separate knowledgeable people from beginners. The duties we expect this DBA to do are: -Design and implement new Informix databases for OLTP, Including logical and physical database design, partitioning, performance tuning, backup and data replication to in-house and off-site servers. -Participate in the migration of current data in Foxpro to Informix. -Work closely with Networking Dept. to fine tune OS (Solaris) where Informix server runs.