Use some DBus debugger, such as d-feet or QDBusViewer to push and poke the DBus interfaces:
- Start by calling getAllContactUIDs, which returns a list of all valid uIDs
- Take some or all of those uIDs and call getNamesByUIDs to see who in your contacts book is being represented
- Take some or all of those uIDs and call getPhonesByUIDs or getEmailsByUIDs, and verify that the data coming back is the correct data as you have entered it in your contacts database
- Make up some uID and enter it into getNamesByUIDs, getPhonesByUIDs, and getEmailsByUIDs to verify the plugin is able to handle 'bad' data