How to tell if a Webform field has been hidden by conditional rule
Assuming you know the numeric id of your field (for example let's say it is 5)

If $submission->data[5] is an array, 
then the field was submitted. The array may contain an empty string, which means the field was left blank.

If $submission->data[5] === NULL 
then the field was not submitted because it was hidden by a conditional rule, or was excluded from the form for another reason (e.g. the user did not have permission to see it).