Its quite common that we need to have the user logged in session to be verified in all pages of a website. So the best practice is we have to write it in one place and let all pages call that (automatically, ie, no exclusive calls on each page)
Either we can use the BasePage technique or can implement a custom written HttpModule that will be called on each postback.
1) Base page technique is very simple. Write a Basepage.cs class and in base page you will inherit the System.Web.UI page, where you can check the session existance. Then all of your project related pages will inehrit this basepage class instead of the System.web.UI namespace