Yes, in general you can't say "what version of X am I using?". You have to ask: "what version of X, the derivation Y is using?". You can see the contents of pac

Yes, in general you can't say "what version of X am I using?". You have to ask: "what version of X, the derivation Y is using?". You can see the contents of package X on your system, there's no index provided online for that. Also "contents of package X" is too general for nix, it's "contents of package X built with inputs Y", you may have the same package X built in different ways in your nix store. It's not impossible, there's the need of a service that provides information on what packages provide a certain file.