Until you have hit elite athlete levels of training, which is 4+ hours per day every day, you really have no fear of this. For 99.9% of the worlds population, overtraining simply doesnt exist. You might be sore. you might be tired. You might have less energy than the day before. This isnt because you are training to much. Its because you eat like crap and arnt getting enough rest.
With proper nutrition, timed appropriatly and mixed with enough rest you are perfectly capable of training like a beast. Ever have a protein shake right after a workout, and marvel at how your not sore the next day? Ever get a real good nights sleep, and find you have endless energy?
This game is more about how you eat, when you eat it, and how much rest you get than anything else. If you eat nothing but carbs and deep-fried bacon, then sleep for 5 hours because you exist on coffee, you will not become fit. Your body cant recover from your workouts because it has no materials to build with, nor time to do so in.
There is no such thing as over training. Just poor nutrition and sleep habits.