Domination depends on two factors: 2/3 of the landmass (including coast but not sea) and 2/3 of the population.
There is a utility program "mapstat" that you can downlaod and use to track how close you are to the domination limit.
You can use the histographic display "Power" and "Score" graphics as a rough approximation of domination levels but this is not very precise when you get down to yourself and just 1 or two ther powerful opponents.
Any time you have a power or score bar that is greater than 50% of the screen, you should probably consult mapstat. The displayed score values are based on teh average of the current score with all your previous per turn scores and these numbers include territory and population factors.