How to write a powershell script which will display the customization done on a sharepoint site - customizations like custom features, custom workflows, custom site def, infopath form etc...
I think this is far to complex for powershell - or you have to invest a lot of time.
I would recommend you to give http://www.spdockit.com/ (paid product, I am not related to the company) a try.
Here is the free version available on CodePlex
This should create a visual snapshot of your SharePoint Farm Configuration, Components and Usage as a single HTML Poster.
Now you have to do some more detailing into to filter custom once :-)