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. There is a distinction between public domain and freeware, but Wikipedia's list of commercial video games released as freeware is probably where you'll want to look for freely available old commercial games.
To put it simply, it isn't.
There are a very, very few games first published in the US prior to 1977 without a copyright notice, or first published in the US prior to 1989 with neither notice nor copyright registration. Some of those early games are things like Spacewar for the PDP-1 or Star Trek for the Sigma 7.
For the most part, games published in the United States are copyrighted for 95 years from publication. This means that the first commercial non-arcade video games will enter the public domain in the US on January 1, 2073, when copyright on the early Atari 2600 games expires.