My colleagues in the hard sciences (and perhaps in the social sciences as well?) are usually able to calculate in an extremely empirical fashion their "impact on the field" (as our P&T committees require) because their disciplines publish almost entirely in journal articles, which are fed into the citation indexes (our library subscribes to Web of Science; maybe there's another one too?). So they can track how many times a particular article of theirs has been cited by someone else.
That doesn't really work for those of us in the humanities -- sure, there is a "humanities" section of the Web of Science database, but it's woefully inadequate as a measure of our citation rates, because many humanities fields rely on book publication as the supreme measure of academic and professional success. I don't know of any database that tracks footnotes in books -- it seems an unwieldly and unlikely undertaking.
So there's a certain sense of unreality about my publications -- they're circulating out in the world, at least in theory -- but I'm not kidding myself -- the numbers of people who read the kind of journals I'm published in is fairly small. So whenever I hear from someone that they saw an article of mine, I'm pretty stoked. (They don't even have to have read it -- just seeing it in the table of contents is good enough.)
Obviously, if someone else published an article about something directly related to my area of focus I might see it and therefore know if they cited me. But so far I don't think I've stumbled across any that way. (Again, I'm pretty realistic about my limited "impact" on my limited sub-field.)
But I had a great experience of synchronicity this past weekend. The conference I was presenting at involved scholars from two disciplines. I met Professor D , who is in the Other Field, early on in the weekend (it was a small conference, very good for the chatting and networking). At the closing dinner party, he told me that he'd been in his hotel that afternoon reading a new book in his field by Esteemed Scholar -- and had happened to read a chapter that cites something I published! Had he read that chapter before meeting me, or a few days after the conference, I'd never have known. Especially since this book in Other Field isn't something I'd be likely to pick up or read very carefully if I did.
It just goes to show that you really never know who might see your work...