I normally look to see if the steel lamination plates have begun to separate. This tells me that water has gotten inside and the windings will have shorted out against the steel plates.
I had a ballast with this exact symptom and when I tested it, I got a spray of fireworks and tripped the breaker for the entire second level of my house. (Much to the dismay of my family)
I now use an RCD when testing and will eventually get around to purchasing a Megger Tester to test ballasts that may be on their way out.