For me it comes down to activity level in all areas. When manic I usually find that I'm suddely a very involved member of about a dozen different discussion forums, have lots of story ideas, start lots of different projects and believe I will complete each and every one of them.

When I come down I suddenly find I can no longer keep up with all the forums and other things to which I have committed myself, don't complete any of the projects I have begun, and begin to sever contact with all the (literally) many many people wiht whom I have established acquiantances until I am alone with my own dire thoughts.