How to check if file exists via URL

[eluser]codex[/eluser]
When you try to get a file that doesn't exist you get the 'failed to open stream: HTTP request failed! HTTP/1.0 404 Not Found'-error, but how do you prevent this? I'm using file_exists to check if a file is present, but it chokes on the 404.

For instance

Code:

http://img.youtube.com/vi/XgdmsJsOKJc/2.jpg

is valid, while

Code:

http://img.youtube.com/vi/jksfhjhfjfdjsfjdsjhjhfd/2.jpg

isn't.

How do you check the validity??