You could use isfile
if(isFile("Add-Ons/lolcs")) { //blah } I think is how it's used, but that's stupid because if you use that and the file isn't there, then when the file is created, and you try it again, it will still return false until the server restarts.
Another way, is to read all of the file and check if it's blank, if it is, presume the file doesn't exist. A stupid method, i admit, but it's more effective than the other one >_>