You cannot tell from the files, and neither can your AI.
A modification date tells you which was touched last, which is frequently the wrong one — you opened the old plan to check something and your editor saved it. A filename tells you what somebody hoped at the time they typed it. Neither records the thing that matters, which is the moment you decided one superseded the other.
Every tool that writes for you produces another artefact and none of them retire the last one. Ask two assistants the same question a month apart and you have two plausible documents, both current by every signal a filesystem can offer.
Retrieval then makes it worse rather than better, because search returns whatever matches the words best — and an abandoned plan often matches better, since it was written in more detail before anybody lost interest.
Canonly marks one version current and the rest replaced, keeps the older ones readable rather than deleting them, and ranks what your AI retrieves by that status — so a superseded document loses to the current one even when it is the closer match.