Well, it depends on the difficulty, and as you allude to the victory condition. Some tools, notably CivAssist II, can help with telling if you are on pace for victory.
But more generally, and in-game, I find the score screen (F8, particularly the Power graph) to be a good sanity check, and the Diplomacy screen to be quite useful. I guess a few guidelines I'd have are:
- Keeping up with other civs in number of cities. You don't necessarily have to have more, but if you're more than a couple behind, that's probably a bad sign.
- Keeping up in the tech race. I'd say "in the first 25% to the Middle Ages" is a good benchmark. But you can use the Diplomacy screen to see what, if any, techs can be traded, and for 20K you'll want to be up-to-date most of the time.
- Generally speaking, at least one worker per city to keep up in tile development. Might not happen immediately due to a settler focus, but definitely should be, say, halfway through the Ancient Age. Even just building roads can vastly improve expansion times.
For 20K in particular, of course, Wonder completion is important, and that ties in with tech. I tend to build Libraries early and often, but strategic tech trading can also help immensely in keeping up.
On higher difficulties (Emperor and above) it is probably not realistic to keep up in cities, and keeping up in tech will be more difficult. But Emperor is also the limit of where I can give practical advice.
