There's a lot of games out there whose original creators have long gone out of business. How (if at all) do these enter the Public Domain, and how can you tell? Are there other kinds of abandon ware?
Just because the title has been "abandoned" doesn't mean that you can download it without worrying about copyright.
Video games in the public domain are very rare. It takes at least 50-70 years for a game to enter the public domain, and even the oldest games aren't really this old (Tennis for 2 is all). It doesn't happen automatically if the company goes out of business or anything like that. The only other way a game can enter the public domain is for the copyright holder to explicitly state that it is in the public domain, and that doesn't happen too often.
So, effectively, the answer is that the game is almost certainly not in the public domain. Count on it still being under copyright.
(I am not a lawyer, do not take this as legal advice)
At this stage, the game has to be released into the public domain by the copyright holders. The largest site that keeps track of that sort of thing at the moment is Liberated Games.