A possible solution might look like this

If you expect people to figure out whether a file is safe before "launching/opening" it, then you are expecting people to solve something similar to the "halting problem" (which I heard is very hard).

Thus I propose that:
1) compliant programs be allowed to _request_ what they want to be able to do (by either using a finite and manageable set of standard sandbox templates, or in special cases a custom sandbox template - which can be audited and digitally signed by 3rd parties).
AND THEN
2a) The user be asked whether the request seems reasonable e.g. Fun Screensaver requests "Standard Screen Saver" privileges vs WARNING!! Fun Screensaver is requesting "Full System" privileges!
AND THEN
3) If approved, the operating system then enforces the requested template, so the program can only do whatever possible within the template sandbox.

Do note there's also:
2b) The request is silently approved if the OS has been told to remember the user's prior approval of the program and template (and the alt/whatever key was not held down while launching).
2c) The request is silently approved if the program and requested template is signed by trusted parties (e.g. OS vendor), and the alt/whatever key was not held down while launching.

I have proposed this concept before to Ubuntu and Suse, see:
https://bugs.launchpad.net/ubuntu/+bug/156693

It'll be hard to implement, but I suspect it's easier than getting people to reliably solve the "halting problem", and in typical cases solve the halting problem without even being able to look at the actual source code.