One way is checking your heart rate - elevated heart rate can be a symptom of overtraining.

Usually for me it's when I start to dread my runs. if I start to suspect overtraining(kinda like you are)usually I will finish off the next scheduled workout or maybe the week than plan a full blown easy week. I go real easy on volume during the recovery week and I'll plan a race at weeks end. By midweek usually I'm getting itchy feet and the race helps me to 'want to stay fresh" so I'll contune with light training. Without the race as a motovator sometimes I jump the gun and I'm flat again within a few days. It's great going into a race all fresh and feeling fit. Even though you don't push it in the race usually the feeling of finishing strong is enough to ensure that you did the right thing. Than it's back to work.