It's not the results of the script that I'm referring to, it's the contents of the script. If I hand you code that can look at your system and determine something about it, you can look at what the code is doing and identify what it is looking at, which tells you where the vulnerability is.
It becomes moderately harder if it's compiled code, but still not very difficult.
So make the code query an opaque EC2 api, instead of testing the machine. You could still find a machine that is vulnerable and one that isn't and attempt to find out what the difference is, but that's a much harder task.
True, and if that's the case, my concern is resolved.
That said, if they were just checking software version or querying an Amazon API endpoint, I'd expect them to give out a tool or URL you could use that would give you the state for all your systems at once, rather than a script that you'd run on the machine itself.
Unless the fixed version of Xen exposes some kind of signal to its VMs that says "Hi, I'm not affected!" - which then could be read by this script. I don't know if and how that's possible with Xen though.
It becomes moderately harder if it's compiled code, but still not very difficult.