can AS see if there is a text file on the server...

[cancerinform]

All you need to do is call the text file and load it. If it cannot be loaded it does not exist on the server.

var myFile:LoadVars = new LoadVars();
myFile.onLoad = function(success:Boolean){
if(success){
trace("File exists");
}else{
trace("file does not exist");
}
}
myFile.load("file.txt");