CheckOutInfo option outputs in the XML the list of the current check-outs of the object - i.e. who has the object checked-out, since when, for which access classes and on which client specification (if it is a persistent check-out). The check-out information is rendered as a 'nav:checkOuts' element, that contains one 'nav:checkOut' sub-element for each check-out. If the object is not checked-out the 'nav:checkOuts' element will be empty.
For example, if the 'img' object with id 5911 is currently checked-out for access class 'Normal' and checked-out persistently for access class 'Content', executing ../do.ashx?cmd=objsnav&ids=5911&options=CheckOutInfo&max=5 produces:
<img nav:objectType="img" id="obj5911" . . .xmlns:nav="http://www.teradp.com/schemas/GN4/1/ObjNav.xsd">
. . .
<nav:checkOuts>
<nav:checkOut Code="Content" Time="2011-02-08T21:45:00.683Z" UserId="1705" ClientSpecId="3761">
<user idref="obj1705">
<nav:refObject idref="obj1705" />
</user>
<clientSpec idref="obj3761">
<nav:refObject idref="obj3761" />
</clientSpec>
</nav:checkOut>
<nav:checkOut Code="Normal" Time="2011-02-08T21:48:05.073Z" LoginGuid="2b120275-8c1b-4842-98bb-a460e29f9ab0" UserId="1705">
<user idref="obj1705">
<nav:refObject idref="obj1705" />
</user>
</nav:checkOut>
</nav:checkOuts>
. . .
</img>