A good way to determine if you are experiencing overtraining symptoms is to test your resting heart rate. You have to determine a baseline when you know you are rested, but after that it should stay essentially the same. If you see a sudden rise (3-5 bpm), then you are in a metabolically "over-reaching" state, meaning you are stressing your body more than it can handle. In the short term, this is good, you need to overload your body to progress. However, if it stays elevated for the remainder of the week, you probably need a back-off week.
There is a fine line between overtraining and just plain working your ass off. I tend to err on the side of "overtraining", because as long as you are paying attention to your body and backing off properly, you should be able to recover. If you can beat down your body so badly in 3 weeks that you can't recover on the 4th week, you've got some great work ethic! Now obviously injuries, etc. are a different story.
One final note, overtraining is usually a result of volume and not intensity (if you are stressing your CNS too much you will be able to tell because your performance will start declining), so don't neccessarily throw out the HIIT, just cut down on the volume at first and work up from there if that makes you more comfortable.