«

[The Fifth Horseman]

If the game is abandoned by it's former company, i.e. they neither support it nor claim any copyright to it anymore, then you can download this game and / or offer it for an upload here on Abandonia, means it is legal.[/b]

Close, but no donut!
1. Not "former company" but "copyright owner". Regardless of what you think, that is not the same thing.
2. Not "nor claim any copyright to it" but "do not actively protect their copyrights to it". Again, two different things.
3. Not legal. Never legal. Quote from the site FAQ:

Since the software is no longer sold or supported, the copyright holders are not directly harmed in any way. This is why abandonware sites are, for the most part, ignored by the law. The distribution of copyrighted software however is, and will allways be, illegal![/b]

Thus: not likely to get you in trouble, but not quite legal. Not warez either (there is a number of distinct differences between warez and abandonware).

If I upload my 'back catalogue', how do I know about the legality of what I am uploading? Also, how is the legality of downloadable games determined.[/b]

The way we determine it according to Abandonia's definition of Abandonware (along with the definition itself) is described in this thread. Furthermore, the same thread contains several pointers on the subject of determining the status of games not yet present on our Oldwarez List.