how to check if file exists on server

i want to check if a file exists on a server, but not to download it...
first i want to see if file exists and then to start to download it, if necessary...
the server may be a HTTP , a FTP or a simple FILE server...
hope you can help me...
thanks

try to send a File Request to the server. If you get an 404 Error, the file doesnt exist, or was not found on the requested Path.