When I design and make a game, I have a checklist of all the things I need to do for it. I usually re-do the list weekly, as things get completed, etc. It has three sections: Fixes, Functionality, and Features. Note: This list can be multiple pages, but each page has these three sections, so the distribution is the same.

Fixes are for things that are game-killers. That is, huge bugs that need to be worked on before anything else can happen. Functionality is for things to make the experience of the game more complete for the features I've already introduced. Like, if I already have a menu feature, making sure I have a back button or something attached is a Functionality item (maybe it's something I forgot-- it's not going to crash the game, but it's still required). Finally, the Features section are for those things that I want to include in the game in the future. This is the most stimulating stuff, obviously.

In the beginning, when I'm still dreaming up the game and expanding the scope, Fixes and Functionality take up the top half of the paper while Features take up the bottom half. Plenty of room to dream (I tend to make each section two-columns, since each checklist item is pretty short, just a note to myself, and I write small).

However, after each successive week, I make the Features section on each page shorter by about a line or so. This means that, after about 10 weeks, I'm having to make tougher and tougher choices about what features I can still add. Doing this leaves plenty of room in Fixes and Functionality for the 'polish' of the game and having the space to write it motivates me to do that sort of thing more.

Once I get down to a Features section of three lines or so, I see the light at the end of the tunnel and the scope of the game has pretty much coalesced. I usually say to myself, "These features, while they would be nice, are just too much work considering I already have the game so close to being ready" and those features are set aside for updates and/or future versions. Then I just make Fixes/Functionality lists until the game is ready to ship.

That's how I determine when a game is done.