If this is coming from a form you can use this code to add the nonce to it:
// Create an nonce, and add it as a query var in a link to perform an action.
$nonce = wp_create_nonce( 'my-nonce' );
<form action='youraction?_wpnonce=<?php echo $nonce?>'>
<!-- Form Contents -->
</form>
Then you can add this to where you are processing the form:
$nonce = $_REQUEST['_wpnonce'];
if ( ! wp_verify_nonce( $nonce, 'my-nonce' ) ) {
die( 'Security check' );
} else {
// Do stuff here.
}