silverlight role keep state

i have a silverlight project

login.xaml->main.xaml. user input username,password to login

if login success ,the program redirect to main.xaml

else stay in login.xaml

my question is that how to rerify the current user identity is valid again when user press f5 ?

i can keep the current user info in cookie or IsolatedStorage,but how can i detect user refresh

because i want to rerify the user role info again where refresh