To help you anticipate when these events occur, Sky & Telescope publishes a detailed list during those months when Jupiter is viewable. The times are computed by the prestigious Bureau des Longitudes in France.
The Bureau got it right, but due to a production error the incorrect table appears on page 58 in our May 2008 issue. (The accompanying diagram on that page, showing the moons' motions throughout the month, is correct.)
And you can also use our free interactive utility to help you identify the moons of Jupiter in your telescope's eyepiece.