If it is frozen, the corn will start off as a crayola yellow, and work into a more vibrant appealing yellow. I know that doesn't help much, but once you feel it starting to get tender, you got about 4 or 5 more minutes. You really can't burn corn unless you let all the water boil off.
Another indication is that it get a little bit heavier from the water it has absorbed, but it's not as easy to tell that.
edit: Okay, cob. Use a fork or spoon, push on the corn... if it feels nice and soft give it a few more minutes then it's good. When I say soft, just think "Could I really pull this off with my teeth?"