How to determine which certificate that client uses?

[Virginia]

Hello, I have a Web Server which uses Certificate Authority(CA) and SSL.

One computer with client certificate can browse web pages of this web
server;
Another computer without client certificate can't browse web pages of this
web server,
and it shows error message.

Are there any ways for asp.net and asp to catch this error message,
and doesn't let user see this error message and telling them why has this
error message?

And can asp.net and asp web page determine which certificate that user uses?

Thanks.