The trick is to sneak a peek at the revolver after spinning the cylinder. If the bullet is in the cylinder which will come up next, say something like “Just one more spin for good luck”, spin it again, and look again.
You do have to know which way the cylinder will turn, though. All revolvers (ok, ok, there are probably rare exceptions, but…) turn by one cylinder as the hammer is cocked, but some brands turn clockwise and some counterclockwise. Colts turn one way, Smith and Wesson the other. I’ve read (I don’t recall where) that people have died because of this difference. But if you ever have to persuade a bunch of ignorant Russians that you are a crazy man who is not to be messed with, this is a great way to do it.
(Link to column being discussed: http://www.straightdope.com/columns/991022.html - Jill)
[Note: This message has been edited by JillGat]