Sleep is the number one indicator for me, for both ends of the spectrum. I know I'm slipping into hypomania first when I start staying up all night because I don't need sleep and I spring up early in the morning (which I NEVER do otherwise -- I am not a morning person). Starting crazy projects is the next sign -- usually it's staying up all night working on them! Also, not being able to sit still or concentrate. Once I reach the point of pacing all night, the rest is history...