On the network proxy, there are 2 icons for "edit proxy" or "delete proxy". There should also be a "test proxy" button to test whether the proxy settings are valid. The "test proxy" button could open a form consisting simply of a textbox in which to enter a remote URL and a "Go" button to attempt a get. The results should return success or failure to reach the remote URL along with whatever other useful diagnostic information could be retrieved in such a test.
Likewise the repository proxy connectors should have a "test" button in addition to the existing edit, delete, up, down buttons.