Hello.
It depends on what version of Kentico you use, on your development model, and also whether you mean On-line form web part or a widget. For example, for Portal engine web parts, you can find their usage (a list of page templates) in the Web parts application, on the Usage tab.
If this did not help, can you please clarify the version of your Kentico project and its development model? Thank you.
Kentico Support