Can I detect the presence of ActiveX controls, like Flash, from ASP?

If you are using Flash, you should see the following URLs: 
 
    Microsoft: Info for Developers about Cha... 
 
    Macromedia: Flash Player Detection 
 
Back to the question: can I detect Flash from ASP? Not with ASP alone. You can use client-side code, to some extent. Here is an example for Nav3+ and IE 3.02+ that detects Flash 2.0: 
 

<%      ' This ASP section pre-determines which      ' script to send ... VBScript or JavaScript        a = lcase(request.servervariables("http_user_agent"))      if instr(a,"msie")>0 then          if instr(a,"98")>0 or instr(a,"95")>0 or instr(a,"nt")>0 then              ie32="true"              ' IE 3 or greater on 32-bit          end if      elseif instr(a,"mozilla/3")>0 or instr(a,"mozilla/4")>0 then          if instr(a,"opera")<=0 then              nn="true"              ' NN3 or greater          end if      end if        if ie32 then  %>        <script language="vbscript">      if scriptEngineMajorVersion > 1 then          on error resume next          FIn=(IsObject(CreateObject("ShockwaveFlash.ShockwaveFlash")))          if FIn then              msgbox "Flash Installed!"          else              msgbox "Flash not installed."          end if      end if      </script>    <%      elseif nn then  %>        <script language="JavaScript">      FIn = navigator.plugins["Shockwave Flash 2.0"];      if (FIn)      {          alert("Flash Installed!");      }      else      {          alert("Flash not installed");      }      </script>    <%      end if  %>

 
Of course instead of alert and msgbox statements, you would document.write <object> and <embed> tags or alternate content, depending on the outcome of your test. 
 
If this seems like too much work, you may consider looking at BrowserHawk which allows you to detect all kinds of client-side plug-ins and controls from ASP.