Mac storage basics · Finding big files, Downloads and the Trash
How do I find files I haven't opened in a long time?
Short answer: In Finder press Command-F, choose This Mac and search by Last opened date (optionally with File Size). Rarely-opened files worth keeping are good to move to an external drive.
Finder can search by when a file was last opened: press Command-F, choose This Mac, set the criterion to Last opened date, and pick is before or within last with a date range. Add a File Size criterion to see only big ones.
Old isn't the same as unwanted — tax records and archives are rarely opened but worth keeping. Those are good candidates for an external drive rather than deletion. Storage Studio's Storage Profile separates what you actually work on from everything you store, which makes this easier to judge. See moving folders.