I suppose I might mark off where I started, but I don't. And it would feel funny. And Neil Gaiman's observation about how you can't tell whether something was written on a good day or a bad day applies very quickly: you glance back over the pages, and it blurs together. (Must be worse for those writers who make it a rule to stop in the middle of a sentence.)
Even if I did mark it off, there's the question of how much I wrote. There's variation in how many words go on a page. Stichomythia has its advantages for prose dialog as well as for that on stage, but it does leave a lot of white space. Action, description, decision-making can take up large blocks of text. . . . I've wondered about how people who do NaNoWriMo by hand manage to keep count.
But the real danger is that plopping down a sentence or two is not enough writing to really get the story done, which is, after all, the point of the matter. If you can't determine how much you have written you can't really tell whether you are writing slowly.