How to determine in a client app if the server requires a client certificate

Hi,

I'm using the HttpWebRequest object and connecting to various servers, one of which is setup to require a client certificate.

My question is how in the client app can I determine whether the server has asked for a client certificate. And also what the accepted CAs are. My research has led me to understand that all this communication is handled in the SSL handshake, but how can I get access to it? Because I'd like to do as in IE, show a UI allowing the user to select from a list of valid certificates from the store.

Thanks in advance