Actually there is no way to identify by one field that this project was created using that template.
You may assume that by Enterprise Project Type, but again there is a way to change EPT for existing project.
The only way I know is to compare a template and a project by tasks, by structure, etc. it works like finger print recognition: you select several points which the most probably tell that the project was created from the template.
The worst case I can imagine: a user creates a project by a template and after that removes everything from the project. Formally the project was created by the template but in real there is nothing from the template left in the project.