The network manager must know the basic concepts of networking and IT. Some of the questions can be;
1- What are different network typologies?
2- What do you know about Switches?
3- What do you know about networking tools?
4- How to configure a Local Area Network (LAN)?
5- How to configure Internet on Local Area Network (LAN)?
6- What are the steps to make your network healthy and speedy?
7- What can be the maintenance plan for the network safety?
8- What are the good qualities of a manager?
Anyone who knows anything about network technology will probably know more than you do. I would suggest that you stick to exploring their management skills and have one of the technical staff ask them questions about the technology. You can then get a realistic assessment from them and combine it with your impression of the candidate as a manager to determine who will be the best fit.