If you train with power and use the TrainingPeaks then there is a very useful metric that can help quantify how much you are over-reaching, while Strava has its own equivalent.
On Training Peaks, Training Stress Balance (TSB) quantifies the amount of training you have done over the last seven days (Acute Training Load or ATL) compared with the amount you have done over the last 42 days (Chronic Training Load or CTL).
A positive TSB score means that your training over the last week is less than you have averaged over the last six weeks. Therefore, you are well rested. A negative TSB means you have done more over the last week than your training average for the last six weeks and you are starting the process of over-reaching.
As a rule of thumb, anything below -25 and its time for a break, anything more than +10 and it is time to get back into hard training. However, these numbers do vary greatly from rider to rider, so it's important to get to know your own personal range to find out where you work best.
If you use Strava and are a premium member, then the Fitness/Freshness formula works to similar affect, and is based on either your power or heart rate data. The Fitness/Freshness chart generates three numbers which represent the rider’s current fitness, fatigue and, crucially, form.
Put simply, fitness is the equivalent of CTL, freshness is the equivalent of ATL and, put them together, form is the equivalent of Training Stress Balance - or the difference between your fitness score and fatigue score. As a result, it should help you strike the balance between training hard and recovery. More here