Assuming the question is about wp_get_attachment_image_src.
It could be also about wp_get_attachment_link, but, although related, this analysis doesn’t includes it.
Got aware of this issue answering this question: How can I view all WP generated thumbnails in Media Manager?.
Refer to it to see a working code regarding the issue on this Question.
And it lead to this WordPress forum topic.
The function wp_get_attachment_image_src( $attachment_id, $size, $icon ) returns an array containing:
[0] => url
[1] => width
[2] => height
[3] => is intermediate
If [3] is false, the original image data is returned.
Both wp_get_attachment_image_src and wp_get_attachment_link rely in the function image_downsize, inside /wp-includes/media.php.
And that’s where this 4 items array is being returned.