Geomagnetic activity is measured using the Kp Index from 13 stations around the world. The Kp Index is a number from 0 to 9. If the number is 5 or above, your chances of seeing a geomagnetic storm that produces auroras are great.
The Kp Graph above shows the amount of magnetic activity that had occurred for May 2003. You can see May 29th was not as good for viewing auroras as May 28th and 30th.
You can get the current Kp Index for geomagnetic activity here. The page is updated every fifteen minutes.
