A key indicator for Overtraining Syndrome is the athlete's emotional state. There are various approaches to measuring emotional state, mostly based around questionnaires. Profile Of Mood States (POMS) was developed in the early 1970s for psychotherapy and counseling, but rapidly became popular for evaluating Overtraining Syndrome. POMS has over 60 questions, which is a little much for many recreational runners, and it is not readily available. There is also the Recovery-Stress-Questionnaire for Athletes (RestQ-Sport), which has a 72 and a 56 item questionnaire plus software to analyze the responses.
A Pragmatic Mood State Questionnaire
There is a shorter approach which has only 6 questions which are rated 1 = strongly disagree; 2 = disagree; 3 = neutral; 4 = agree; 5 = strongly agree. Add up the answers to get a number between 6 and 30. A score below 20 indicates rest is required. Any questionnaire should be taken at the same time of day and should be tracked over time.