Many people suggest checking your morning heart rate to check that you are fully recovered from heavy training sessions, for some information on how to do this, see here.
My problem with using heart rate for anything is the natural variability in your heart rate from day to day. Jeff Gaudette discusses how he does it in this article, but even he says that we shouldn’t be concerned by a 3-4 beat difference – but that seems like quite a lot to me!
I have always preferred to use RPE when starting training as a gauge, from this I an feel whether my planned training might be too hard and adjust the intensity accordingly. The various studies cited suggest that my feeling is right and it is actually stated that 4 separate studies found no correlation between resting heart and overtraining.
So what is the best way to figure out if you are overtraining? How about this quick test for cyclists that you can do at the start of your turbo trainer session? Admittedly it might not be very helpful for runners but perhaps you could use the same method outlined here or use RPE during your first interval to help you figure out what you should be doing in training.