There's lots of ways to monitor training loads and how you are coping with and adapting to them, which is really important in knowing much is the best (or 'right') amount of training to complete. This can range from a simple question like "How are you feeling?", and watching body language...which can often tell more than is mentioned in a verbal response...to data based metrics based on a range of measures (see below).
One well known sports scientist talks about the "hair in the porridge" test, observing that athletes bent of their morning bowl of porridge are likely more tired than those who are happily chatting. Simple, but very effective!!
Training Load Metrics
The key is having a way of measuring the stress from individual training sessions, which can be assessed based on duration, distance, avg pace, HR, TRIMP (TRaining IMPulse), training pace or power (if you use a running power meter) against a baseline measure, blood lactate measures and simple Rate of Perceived Exertion (RPE).
Perhaps the most readily accessible metric is using RPE (1 to 10) multiplied by the session duration (in minutes). Eg, a moderate intensity session (RPE = 5) for 50 mins = 250 "units". Eg, a race (RPE = 10) for 20 mins = 200 "units". The key is being consistent in how you measure RPE.
Acute Training Load (ATL)
The real value is measuring training load is to monitor it over time to see trends and patterns, and relate them to how your fitness and performance. A commonly accepted time-frame for assessing acute, or short-term, training load is 7 days, on a 'rolling' basis, that is, the previous 7 days from any point in time.
Your ATL score in "units" will give you a reference point you can compare to other 7 day periods to know how much training workload you are completing. In general, you want to avoid dramatic changes in ATL, and in some cases, maintaining a stable ATL is good.
Chronic Training Load (CTL)
The time-frame for assessing chronic, or long-term, training load is 42 days, also on a rolling basis. 6 weeks is often regarded as a period of time in which you will see adaptation and progress to a training stimulus. For example, if you do a block of speed / power training, after 6 weeks you would likely see some measurable improvement.
CTL is good for seeing how your training load varies over a year, and various goal events you might build up for, and then recover afterwards, etc. The CTL curve will have peaks and dips that are more gradual than an ATL curve, given its long-term nature. And as with the ATL, making sense of CTL will come through recording and monitoring it over time, while not forgetting the value of subjective "how are you feeling" questions. It all starts with consistently recording a training load metric!!