API Contact>DuplicateCheck Not Working?
Here is a piece of code that works :

$res = civicrm_api3('Contact', 'duplicatecheck',
  array(
    'match' =&gt; array(
      'contact_type' =&gt; 'Individual',
      'first_name' =&gt; 'Jane',
      'last_name' =&gt; 'Doe',
      'email' =&gt; 'jane@example.org',
    ),
  )
);


Return :

Array
(
    [is_error] =&gt; 0
    [version] =&gt; 3
    [count] =&gt; 1
    [id] =&gt; 810
    [values] =&gt; Array
        (
            [810] =&gt; Array
                (
                )

        )

)


Note that :


contact_type is really important because it help the api choose the proper rule and CiviCRM won't give you any error if you omit it
I'm not sure if it's possible to test it from the api explorer because match is an array and i have no clue how to put an array in this context...