First thing in the morning, before getting out of bed, I take my heart rate using a Tanita index finger heart rate monitor. I take it for at least three minutes to find a floor HR. The advantage of taking HR with the Tanita is that you can see your HR dropping so you can keep measuring until you reach a floor. The only exception to the above procedure is if that I might have to pee or [censored] when I first wake up. In that case, I tend to business and immediately lie back down to take my pulse. Sometimes, I can even tell if I have to pee or [censored] before I even consciously know it because my pulse will be erratic and will suddenly begin climbing.
I have found this method to be very reliable and telling and, along with other overtraining signals such as libido, attidude, vigor, appetite, sleep quality, etc., has proven to be a vital part of my training regimen.