You could do this without a script, but it won’t be easier.
You’d have to branch out of the main trunk, split your XML file, and for each item listed, run a Folder Listing set to use the path/filename as a mask. The resulting XML tells you the number of files it found (along with information on each file) so you could do a quick compare against the File count node to see if it’s equal to 1.
I’m pretty certain, however, that querying the OS via FileSytemObject is going to be more efficient than all of that. If you’re comfortable with either jscript or vbscript, I’d recommend simply going that route.