How Can You Tell When You're Sick Enough to Call In Sick?

I'm never really sure.

I woke up this morning with a slight headache, compounding my annoying cough and colds. I'm sure I would have been able to make it to work and put on a whole day's work, albeit probably at a less productive level. But then again, if I rest today, I may be able to put in a 110% productive day tomorrow, whereas not resting might cause me to be unproductive for more than one day.