if you are sore or stiff for more than a day, if there are signs of trauma (swelling, inflammation, fever in muscles & joints), you don't feel rejuvenated and instead are tired and worn out. and many other things
Overtraining - Overtraining Syndrome
There are several ways you can objectively measure some signs of overtraining. One is by documenting your heart rates over time. Track your aerobic heart ...


http://sportsmedicine.about.com/cs/overt...