You can do it as
Page pg = ContentPlaceholderPageName.Page as Page;
if(pg != null)
but i would seriously not recommend that. whenever we see a scenario where master page is referring the content page, we should rethink our design. Always make content page refer to master page. have properties in master page to push data in it. the reason is that the master page should not depend on content page because there will be many content pages using the master page. the reverse dependency is OK and in fact recommended for any master-content page communication.