One of the things I do is to keep track of my resting heart rate. Its actually my sleeping heart rate. After a long ride the body sometimes needs time to rest and will let you know. First the lactic acid buildup in the muscles and even more important for me is the heart rate at rest.

At full recovery, the heart rate will reach its low. After a strenous workout, the heart rate may escalate a bit, say from 55 beats per minutes to 62 bpm.