Deciding what is considered over-exercising can be touchy and confusing for many people. Especially when you consider professional athletes who are required to be in the gym and/or the field for set amount of times (usually the majority of their days)
To decide if you are over-exercising, you should listen to your body during and after your workouts. If you are physically and emotionally exhausted and can barely move, you most likely exercised too much.
Physical effects
muscle strains/sprains
shin splints
stress fractures
too much weight loss
Low body fat percentage
affects hormones and can cause early menopause and osteoporosis
risk of anemia and weakened immune systems
slow metabolism
Soreness and achiness is a sign that your body needs a day off and to rest.
Psychological affects
fatigue/no energy
depression
A great number of people who over-exercise are actually punishing themselves for eating something they think they shouldn't have. The motivation for the workouts should always be considered and if in a healthy mental state, would be "to be healthy."
Social effects
Damage to social life
A very common side effect of over-exercising is choosing to be in the gym instead of participating in social events. This can damage your relationships with your friends and families. You should never feel guilty for missing the gym to have a social life. If you experience this, you are most likely exercising compulsively.