Well, once a new drawing is created, it does not remember which template is is based on. So, there is no way to know that.
In your case, it is not important to know which template is used: user could use the template to create a new drawing, and then create more new drawings based one the first new drawing: are you going to say the later new drawings are based on wrong template? What you need to do, before your form is showing, is to validate the pre-existing content are there, such as layers, text/dim styles.... In this case, you can examine/compare each type of content against a particular drawing (template or not, does not matter here). Hopefully, you only do minimum of this; or you need something like "Cad standard comparison" type of work.