The symptoms of overtraining include:
(i) A higher-than-normal resting pulse
(ii) Illnesses become more frequent and last longer than usual
(iii) Muscle spasms while resting e.g. eyelid twitch etc.
(iv) Shaky hands
(v) Loss of Sleep
(vi) Loss of appetite
(vii) Unexpected and unexplained fatigue
(viii) Unintended weight loss
and MOST IMPORTANT OF ALL
(ix) very slow or non-existent gains in exercise poundages in the gym. That is less than 2 lbs. a week on each exercise. In extreme cases overtraining can lead to DROPPING poundages.
[Taken from the Hardgainer FAQ (
http://www.hardgainers-home-gym.com/hg_faq.html) - a great source of information as well for those who aren't hardgainers]
If you're getting any of these, then you're very likely are over training. Note that it may make sense to temporarily overtrain as long as it's done in terms of overall intensity cycling.