Templates are not used for sending. I am assuming what you actually mean is Email. These are what you assign data to in order to create a job that sends an email to an attached audience.
Your best way to validate this would be via a SQL Query on the Job Dataview. Now this would only give you a six month look-back, but from the sound of things, that should fit in your needs as you are only looking for most recent sends and most recent activity.
Max(EmailName) as Name,
Count(*) as TotalSends,
Max(DeliveredTime) as LastSentDate
GROUP By EmailID
Should get you there and you can assume that if it is not on there, then it has not been used recently or is a brand new template.
I use EmailID as the key in this query because the name of the email can be altered, but the ID of the email will always stay the same. Which is why I have
Max(EmailName) be what defines the name of the email. Any query with
GROUP BY in SFMC requires all 'non-key' (not in GROUP BY statement) to be aggregate functions (MAX,MIN,COUNT,SUM,etc) so I set MAX() around EmailName so it can be grabbed but will not need to change the output. As a note, with MAX, it will pull only the name that is 'highest' in the alphabet, so you will not see any of the alternative names.
If you need to look further back, I would look at a series of tracking extracts that pull your required timeline worth of data. The output you would want is 'SendJobs'. This will be very similar to the