How to get the value in a link field using $_GET

I am using link field module to create a url field for a custom content type. My field name is 'field_my_url' and users can submit a url through this field. Now i am trying to make a custom module to check if the user submitted url is already present in the database or not. I am using uniquefield module and username_ajax_check modules as reference for my custom module development. For checking the url value of a link field, what format should i use? For example, if i want to check for the title field, i can use

 $title = $_GET['title'];

So for my url field, is it the correct format to use-

$f = $_GET[$field['field_name']]; // where $field is used to represent any url field of a given content type.

or $f = $_GET[$field['url']];

or $f = $_GEt[$field['field_name']['url']];

or any other format is required???

Any help in this regard will be much appreciated.
Thanks in advance!!