Septi it's really a matter of common sense.
You can tell it's a bootleg because of the imperfections on the sleeve.
The photo does not cover the entire sleeve, Sean Lennon has been cut out from the photo, as well as the flying machine, so that gives you an idea that this is not an official recording.
Other than that, it's called a GREATEST HITS, however, it includes GIRLFRIEND, which is clearly, not a hit at all, and it says that OFF THE WALL comes FROM SOUNDTRACK OF MOONWALKER, and OFF THE WALL, is not included on the movie. And GET IT? This was a really MINOR hit...
Apart from that...it lacks WAY too much production information, which official releases DO have.
So , ok, it's a bootleg, but a rare one, so you should be glad to have it.