You might wondering if the content you access was already cached on CDN or not. To check cache you can use curl command.
curl -I https://cdn.statically.io/:path_to_content
curl -I https://cdn.statically.io/gh/mrdoob/three.js/dev/build/three.js
X-Cache: HIT means that the content has been cached on the CDN. Each CDN usually has a different indicator, please also check
CDN-Cache if it's listed.