You could include a web bug, although I personally frown upon such things.
Example: 1x1 transparent pixel ... with a url like so:
You could include a token in the URL, and process it when they land on your page:
You could embed a tacking script (google analytics is both pervasive and annoyingly easy to use).
Or serialize the server's request/response headers and store them/data mine them later, and use bogus tokens but keep track of them (they'll have to be unique).
Lots of ways to do it.
I believe "constant contact" offers a solution (possibly free, I dunno).