Check if custom thumnail size image has been cropped to the correct dimensions

$image_attributes = wp_get_attachment_image_src( $attachment_id, $size ); 

Returns an array containing:

[0] => url
[1] => width
[2] => height
[3] => boolean: true if $url is a re-sized image, false if it is the original. 

Even though you pass the $size as parameter, while returning; it returns the actual width and height of the image irrespective of the image being cropped or not. Then you can check for the width and height if these are smaller then the requirement. OR better apply min-width, min-height in CSS or jQuery.