[cbr]

You can already do that.

     good.com:
        <script src="/js/site.js">

     evil.com:
        <img src="https://www.good.com/js/site.js">

Then use the navigation timing api to figure out whether the js was already in cache.

[cbr]

(Actually, you could use the onload event; you don't actually need navigation timings.)