How to do unit tests on a multithreaded server?

Assuming I have a server that extends a thread class and every connected client will be a subthread. This server accepts a new client using socket api.

What is a good way to do unit tests on this server? 
What should the unit tests cover? 
I wonder how to **simulate clients**. 

Update:
I decided to share my code on [github](https://github.com/Jun711/ServiceServer/blob/master/app/src/main/java/com/jun/servicemonitor/ServiceMonitor.java)