Overtraining is a common problem in people who work out often, the idea behind overtraining is that if you put a lot of effort in a short amount of time into one area that it will exponentially boost the muscle production in that area, which is not true. The more you workout an area without break will give severely diminishing returns and will cause injury to the area whereas normally it should just be sore for a day or two.

1. Pain

If you are overtraining yourself you're going to notice aches and pains around your body, more so than any usual workout pains but will go away over a few days if you actually rest.

2. Sickness

If you find yourself unable to shake off a cold for longer than usual your body is putting much more resources into recovering your body from the overtraining and taking energy away from your immune system.

3. Sleep 

Over training increases your blood pressure, alertness, and energy which are not good for getting a good nights rest when you need it the most, which is after exercising. It could be caused by many things however so analyze your intakes before settling on overtraining.

4. Performance

If you are noticing that you feel weaker than you should when going to the gym then that is another sign that you may be overtraining as your muscles havnt had time to catch back up with themselves and have not been given the rest that they need to grow.

5. Appetite

A loss in appetite is a sign that you may be overtraining because it puts a lot of stress on your body as your body tries to keep up with all of the pressure that you are putting onto it by putting less priority on eating and efficient digestion.