A tip that has worked for me is to take my waking pulse each morning. If it goes up or down ten percent of normal, I cut back on exercise that day. I learned to do that while playing around doing triathlons some time ago. Also there is a danger to over-exercising as well; you sometimes will tax your immune system and end up catching every bug you come in contact with. YMMV.
Oh damn. this is why I've been getting small cold and flu the last couple weeks. Normally against these things I'm bulletproof. I don't remember getting sick the last two years until now.