IF current logged in user is 'Author' to a custom-post-type called 'farmers' {
// Do something
} else {
// Do something else
}

<?php
// Global variable for current user ID
$user_ID;

// Create a new WP query object
$my_query = new WP_Query( array(
'post_type' => 'farmers',
'author' => $user_ID
));

// Get all the current user posts from the query object
$posts = $my_query->posts;

IF $posts = 0 {
// Do something
} else {
// Do Something else
} ?>

$query="select * from posts where type='farmers'";
$result=mysql_query($query);
$valid=false;
if(mysql_num_rows($result)>0){
$valid=true;
}


if($_SESSION["id"]==$posts["id"]){
//your code
}else{
//your code
}

if(mysql_num_rows($posts)>0){
// user have post
 }

if($user_session_id == $author_id) {
    // Do something
    } else {
    // Do something else
    }

get_current_user_id() == get_post($your_post_id)->post_author

get_current_user_id() == $post->post_author

$my_query = new WP_Query( array(
  'post_type' => 'farmers',
  'author' => get_current_user_id()
));

if ( $my_query->have_posts() ) {
  // Do something
} else {
  // Do Something else
}