You can see where existing Email Templates are being used in your system by clicking on 'Edit' and selecting the 'Usages' tab.
'Email Template in Processes' shows the total number of processes where the particular template is in use, and in the table below it will break this information down further showing you each individual process where the template is being used, and how many times the template is used within that process.
'Canned Responses in Service Line' shows the total number of service lines in which the Email Template is being used as part of a Canned Response. The table below will break this information down further showing you each individual service line where the template is being used as part of a Canned Response.