Frederick Hatfield recommends blood pressure and resting heart rate, among other things in his book "Hardcore Bodybuilding, A Scientific Approach" as overtraining indicators and I have used these two methods with great success. By success I mean that I can train hard 7 days a week for months on end, only pulling back enough to keep these parameters within range and make continuous progress (as measured by arm size). If you exceed the limit it doesn't mean you are over trained just that you are heading in that direction and immediate action is required.
Go get one of those wrist cuff blood pressure & heart rate monitors (Walmart has them fairly cheap) and put it on your night stand. When you first wake up, before you move around and get things stirring take a measurement. NOTE: you need to be consistent on how you measure and I recommend that you have the monitor at the same level as your heart.
Finding your Baseline:
First you will need to find your baseline measurement. When you are taking your week off from training would be an ideal time to do this. Take these measurements every morning and record them. I record mine in Excel so that I can easily graph them over time. Look at the data and decide if you need to throw any points out. For example, a bad night's sleep will increase your morning heart rate so throw that point out, etc..
Hatfield divides overtraining into two categories: Addisonic and Basdowic because the overtraining symptoms are similar to Addison’s and Basdow's diseases respectively.
For heart rate, measured in beats per minute, I set my limits at 15%. Above 15% indicates Basdowic overtraining while below 15% indicates Addisonic overtraining. If you change the amount of cardio that you do you may have to reevaluate your baseline occasionally - I have only had to do that once because it stays fairly constant. So in summary: for heart rate you want to keep it BETWEEN these limits.
For blood pressure you will want to stay BELOW the limits. You have two numbers in your blood pressure: the systolic (top) and the diastolic (bottom) and they are measured in mmHg (millimeters of mercury) I set my morning limits at +20% for both systolic and diastolic pressures; exceeding either of these indicates Basdowic overtraining. I also set a post training limit of +15% of my diastolic (measured right after training) or 100, which ever number is lower; exceeding this limit indicates Addisonic overtraining.
I tend to fall out on the Basdowic side of overtraining probably 75% of the time and I think that heart rate is the best indicator. It usually gives me the earliest warning.
You need to watch your diet carefully too as not taking in sufficient calories can put you into an over trained state fairly quick. the trick here is to know if you need to back off of the physical activity or increase your calories - assuming you are eating well it means back off the training – i.e. go walk laps for a day.
On that note: I follow Gerard Dente's Macrobolic Nutrition guide from which I calculate my total number of calories for my stated goal (gain, lose, or maintain). He specifies a weight percent of 35-45-20 of protein, carbs, and fat respectively as a balanced meal. Given this, I know my food intake is good so I automatically know that I have to pull back on activity. Legs are usually a real killer for me and most of the time that I either start drifting toward a limit or actually exceed one it's frequently a day or two after hitting legs hard.
Yes, it's a bit technical, but it does draw a very distinct line in the sand that allows you to push to your limits - which is what I like to do. Hope this was helpful.