Sitecore: where are the placeholders for my page!?

You will be asked to aloow some new components on one of your page types and you need to figure out where yo need to make a change.

Take these steps and it would be easy:
1- Find your page which needs change and have a look at the template it is inhereted from.

2- Find the template and see the Standard Values
3- From the View menu just check the raw values and you will see some parts which show a text box Final Rendering then you can see all Placeholders related to this template and all inhereted pages.
4- Go to the Rendering and find the Placeholder and see the allowed components. Now you can edit and add new components.

These photos might help: