The 8 glasses thing is an approximation- how much you actually need varies on a few things. However, if you do drink 8 glasses, you're probably getting plenty.
There are some recent revisions to the 8 glass/64 ounces rule (according to some doctors) that you need 64 ounces of water in general, so you can actually also count the water that is in other foods/beverages. For example if you eat watermelon, lettuce, whatever, the water from those counts too. And while they say you can also count beverages other than water, to be safe you should avoid too many diuretic and caffeinated drinks (those which increase your output of urine) such as coffee, tea and soda. Also be wary of drinking too many fruit juices as they are so high in sugar.
So basically, if you don't get a full 8 8oz glasses a day (say, you get 5) don't fret too much, since you probably got enough elsewhere in your diet.