Turn on IncludeExceptionDetailInFaults (either from ServiceBehaviorAttribute or from the <serviceDebug> configuration behavior) on the server

Define a behavior in your .config file:

<configuration>
  <system.serviceModel>
    <behaviors>
      <serviceBehaviors>
        <behavior name="debug">
          <serviceDebug includeExceptionDetailInFaults="true" />
        </behavior>
      </serviceBehaviors>
    </behaviors>
    ...
  </system.serviceModel>
</configuration>

Then apply the behavior to your service along these lines:

<configuration>
  <system.serviceModel>
    ...
    <services>
      <service name="MyServiceName" behaviorConfiguration="debug" />
    </services>
  </system.serviceModel>
</configuration>

You can also set it programmatically. See this question.

It's in the app.config file.

<configuration>
  <system.serviceModel>
    <behaviors>
      <serviceBehaviors>
        <behavior>
          <serviceDebug includeExceptionDetailInFaults="true"/>

If you want to do this by code, you can add the behavior like this:

serviceHost.Description.Behaviors.Remove(
    typeof(ServiceDebugBehavior));
serviceHost.Description.Behaviors.Add(
    new ServiceDebugBehavior { IncludeExceptionDetailInFaults = true });