Checking Client Certificates C#

Hi all,
I'm developing a SOAP web service which contains many methods. The service is exposed inHTTPS.
Some methods must be accessed only by some clients, so I would like to use Client Certificate to decide if a call is authorized or not.

On client  side I add a X509 Certificate to the service instance (_instance.ClientCertificate.Add....)
How can I check presented Certificate on the server (web service code)?

System.Web.HttpContext.Current.Request.ClientCertificate.IsPresent is always FALSE.
System.Web.HttpClientCertificate is empty...

Thank you