Here is a good site on HID ballasts.
http://www.fortunecity.com/meltingpo...ie/38/id33.htm
From their FAQ file.
How do you know when the capacitor has gone bad?
The lamp will not normally work if the ballast circuit is a CW or CWA.
Testing is the only way to know when the capacitor has gone bad on HX-HPF and R ballast circuits. On HX-HPF and R the amp draw will almost double when the capacitor fails.