I think many of us who have been around a while start to know when we need to unload (I stole that from Frey, because I hate the term deload. I can't bring myself to use the same term that lifters who can barely lift their bodyweight use and implement once a month). Anyway, I have two distinct signs. One is when I don't want to train. That is a very rare feeling. However, it needs to be coupled with something for me to take it easy. It will either be with very sore, almost broken feeling forearms. Or, my hands will be so swollen they feel like concrete blocks at the end of my arms.
What are your signs?