I may have learnt about this here, so apologies if so, but haven't been able to find the original post.

You can embed proxies in your web pages as described at http://crypto.stanford.edu/flashproxy/

That generates a "badge" that you can see, for example [self link] at http://www.acooke.org/cute/ (top left).

And in the latest Chromium on Linux, at least, I am seeing that regularly turn light blue which means that (1) it is being used and (2) they probably need more people to do this (since ideally any one page should be used quite infrequently).

Finally, note that this only makes you an entry point for Tor. So you are not the source of any "disturbing" connections (connections from you go only to another tor node). Also, given the transient nature and low bandwidth, you're unlikely to be used effectively for downloading kiddie porn - it's much more about helping someone in China (say) look at a censored web page (since you're providing an entry point at a "random" and so unblocked location).