Get all Checked in, Checked Out Items in a Sharepoint site

To Get all Checked in or Checked Out items on a SharePoint site you can use the internal 'CheckoutUser' in your CAML query along with IsNUll or IsNotNull options.

Here is a qucik CAML to get all Checked Out and Checked In items from your SharePoint site-

All Checked In Items -
 <query><where><isnull><fieldref lookupid="TRUE" name="CheckoutUser"> </fieldref></isnull></where></query>

All Checked Out Items -
<where><isnotnull><fieldref lookupid="TRUE" name="CheckoutUser">
</fieldref></isnotnull></where>";

To get all the items Checked Out to a perticular user see the two posts below

Using Object Model - Get all Files\Items Checked Out by current user in SharePoint

Out-of-Box - Get all documents Checked Out to current user with Relevant documents webpart in Sharepoint 2010