How to check if a user has any entries they're an author of... in a plugin?
Getting entries in plugins is slightly different from within templates and you should read the details on Working with Elements in a plugin in the Craft docs.

Taking that as a guide, you need to create an ElementCriteriaModel instance which selects the entries you are interested in:

$criteria = craft()-&gt;elements-&gt;getCriteria(ElementType::Entry);
$criteria-&gt;authorId = 10;


Note that the ElementCriteriaModel won't pull disabled entries by default. If you want to pull both enabled and disabled entries, you can add the status parameter to your query:

$criteria-&gt;status = null;


Finally, you need to call find() on the ElementCriteriaModel to fetch the entries matching your criteria:

$entries = $criteria-&gt;find();