Delivery reports can be found in the web interface to your Clickatel account Message Reports menu. The easiest access is via the Last 10 Messages left menu item.
Correct status is "Received by recipient" without further errors. Also the charge can be helpful - each part of a long message is charged as one message, so a two-part message will be charged like two messages and so on.
First enable the SMS Server logging in System Logging. Then observe the logs under Logs Service: SMS and/or Sync Push.