if someone says its in some major key, then it could also mean the 1st load of bars are in that major key, but further along, it is possible for any song to change to different major keys (some of JF’s songs do that).

If you think it is in a minor key, then look up the relative minor key for the major key they claim it is in – that might work.

Also some songs i have, start in a major key, then further along they change to the relative minor key of a completely different major key.

The last resort, is workout what all the notes are in the song by playing each one on the sax and comparing it to the recording.