A report of all checked out documents

The easiest way to get a report of all checked out files is by using powershell script as described in the blog

http://www.sharepointdiary.com/2013/02/find-all-checked-out-files-and-check-in.html

Another option is to use web services see below blog for reference

http://sqlblogcasts.com/blogs/drjohn/archive/2007/11/02/Getting-a-list-of-files-from-a-moss-document-library-using-a-SharePoint-web-service.aspx

Also This blog mentions a very easy way to create a report of all documents which are currently checked out in a site it is listed again here 

1) Go to (out of the box) List  'Content and Structure Reports' and add a New Item.

2) Fill the following values:

    Report Title : CheckedOutFiles
    Resource Id : SmtCheckOutUser
    Resource Id : SmtCheckedOutReportDescription
    CAML List Type : <Lists ServerTemplate="101"/>
    CAML Query : <Where><Geq><FieldRef Name='CheckoutUser' LookupId='TRUE'/><Value Type='int'>0</Value></Geq></Where>
     Report Description : All documents and pages checked out in this site and subsites

3) Navigate to Site Settings -> Site Administration -> Content and structure

4) This gives context to the right-hand window where you can look at views and manipulate content in certain ways.

5) If you click on view: and choose the latest added 'Content and structure' item you can view all the documents checked out in all the document library in the site and its subsite