I distinctly remember arguing that Hunkeford, TX existed. My friends didn't believe it and decided to decide by checking the internet. "Let's look on the web!" If it wasn't there, it didn't exist. Hunkeford doesn't but it turns out a close cousin, Hungerford, does.
I know of the existence of writers by reading their work. But unlike writing, authorship gets lost in many other fields. You can enjoy a chocolate pie at a restaurant and not know who's recipe it is nor who cooked it. Work can have impact but not be traceable by the consumers of that work back to the one(s) who made it so.
Every day I check e-mail, Google Analytics stats, Facebook, Dailymile and Flickr to find out if I existed today and if so, for whom. Oddly enough, the fact that I show up for work doesn't seem to count in my differential calculus. However, if I made a measurable difference at work for someone or some project, it may. Only the area under that curve counts.
My best existence proof comes in the morning when I have slowly awakened without the alarm but before I remember my name or look at the clock. There I am most aware that I am.