If you're a published author, maybe you check Amazon.com sales rank (gosh, of course *I* never do).
You can read this, this, or this.
My book topped out at 500 (that's pretty good) and is currently hovering around 15,000 [update: 50,000]. That means there are 14,999 books selling better than Cop in the Hood at Amazon.com.
What does that mean? Hard to say for sure. But the best guess is that a ranking of 15,000 through the world's biggest book retailer means I'm probably selling about a book a day. And Amazon is perhaps about 1/3 - 1/2 of all sales. In terms of royalties, that's about $2 day.
One way to tell for sure is when Amazon gets low on stock. For instance, Amazon.UK (England) currently has one book for sale. They have had one for a while (or so I've noticed). When it sells, they'll have no books for a day or two. So I can be sure I know the next time I sell a book in Europe. ...It may be a few days.
Doesn't anybody read anymore?
Have you bought my book? If not, please do. It's cheap. And good.