Do you want to check the existence of files in the virtual path of your own application ? If yes,you can use the following code.
Set objFileSystem = Server.CreateObject("Scripting.FileSystemObject")
if(objFileSyste.FileExists(Server.MapPath(<relativ e path of file>))) then
Response.Write "File exists"
Response.Write "File does not exist"
<relative path of file> is the path of the file (to be checked) in relation to virtual directory.
If you want to check the existence of file at a different server, you need to use adodb stream object. Please see the following post.