Check login

I have a context for the single sign on like this:

A) User login on SP. After that user logout from SP and user will be logged out automatically in IDP. This case is OK.

B) If user login on IDP, after that user access SP and user will be logged in on SP. But then if user loggout from IDP, how can SP recognize? I need a method to check if user logged out from IDP or not?

Thanks.