function eppe_check_submission_errors($posted) {
    // all of $_POST is stored in $posted
    if(!isset($posted['your_field_name'])) {
        edd_errors()->add('your_error_code', __('This field is required', 'eppe'));
    }
}
add_ction('edd_checkout_error_checks', 'eppe_check_submission_errors');