Checking if a File Exists

Sometimes it is necessary to determine if a file exists using actionscript, for example when you have a preloader cue, that loads a series of .jpgs one after another, if one of the files doesnt exist then the cue wouldnt traverse any further, you could of course use a timeout in that scenario but why not just check if the file exists before loading it, so that if necessary you can move onto the next item in the cue almost instantly.