Many attempts have been made to find an objective and valid indicator to quantify training load (GPS, heart rate, etc.) but, recent research shows that an accurate, simple and practical method to monitor internal load (the effect of training+competition load+stress and fatigue of the daily life) is the session-RPE proposed by the Prof Carl Foster (see Monitoring training in athletes with reference to overtraining syndrome). This is the method used by AthleteMonitoring.com.
With this method, training load is calculated by combining subjective feedback from the athlete (difficulty of the session as graded by athlete) and objective (duration of the session) markers.
The session-RPE method has been validated for use in many sports with athlete of all ages and levels (more articles here). Furthermore, in addition of quantifying load accurately, additional indicators of training can be calculated.
Session Training load = session RPE x duration (minutes)
Daily Training load =Sum of all Session Training load for the entire day
Weekly Training load =Sum of all Daily Training load for the entire week
Monotony= Daily mean load / standard deviation of Weekly Training load
Training Strain= Daily or Weekly Training load x Monotony
Performance Readiness= Training Load- Training Strain