You know you've peaked too soon when your race performances or workouts in lieu of races are at their best before your goal race and fall off as you near your goal race. The disadvantage of a too early peak is that your performance in your goal race will not be all that it could have been.
The next logical question is how do you avoid peaking too soon? This is a more tricky question to answer. A lot of it depends on your personal makeup as a runner but, generally, shortening your race preparation phase (those final about 8-12 weeks when you're doing the speedwork) would be the most universally effective way of accomplishing this.