What is the correct way to check session status?

Here is what I'm doing now, it doesn't work well at all for catching session timeouts.

 private bool IsLoggedIn()  {      return _service.sessionInfo != null;  } 

I'd like to accomplish two things:
[LIST=1]
[*]create a session valid (do I need to ping the server every 10 minutes?)
[*]reliably check for a timed out session
[/LIST]

Hopefully this is something I'm just missing in the docs.

Thanks for any suggestions,
Steve