I'm not an expert on them, but there are a few things where you can check usually.
1. Indeed a readme or the loading screen which say where it is from
2. Is the track also available on ISI forum - Almost all the legal tracks are available over there, if they are not that is most of the time for a reason, normally someone wants as much exposure as possible
3 Whether you can recognize the track from another game, mostly visable by people who pay attention to modding details, such as graphics, or dimensions etc. But an illegal conversion is also visible by the adverts on the side of the track, or specific details that are only in that certain game. Offcourse you would need to own a lot of games in that case if you want to check that yourself
The Spa track is probably a codemasters conversion done by McNolo who didn't intended it to have it released for the public, the japanese one I have also didn't had a readme or a loading screen and not on the ISI forums, so more than probably an illegal conversion too, but I'm not certain, but unless there is more indication it isn't, we choose not to upload the track.