How can I know if the client browser supports active scripting

You can detect and intercept the capabilities of your client using the namespace System.Web.HttpBrowserCapabilities :
VB.NET

Dim browser As System.Web.HttpBrowserCapabilities = Request.Browser
Response.Write('Support ActiveXControl: ' + browser.ActiveXControls.ToString())

C#

System.Web.HttpBrowserCapabilities browser = Request.Browser;
Response.Write ('Support ActiveXControl: ' + browser.ActiveXControls.ToString ());

For more details Refer: Detecting Browser Types in Web Forms