Use the MasterPage property on the content page. Cast this to the appropriate reference type (your master page's class). Call the methods.
You can also use a directive in the page, and a setting in the <pages> element to specify what type your master page is.
For a truly professional solution, define an interface that your master page implements and cast the MasterPage reference to that type, before calling the methods. That way, you can interchange master pages without having to edit the content page code.