If it is electrolytic, which you didn't say, then practically any voltage will cause it to explode. As Corneo says it easy to try...and it's also a good way to make lecturers/teachers/lab technicians/fellow students rather cross with you - both for wasting caps and making them jump every few minutes :P
Are you sure it was backwards?...I know it's obvious but some people, even degree level students, get that wrong. 9V should be plenty to do the job, unless it's a huuge capacitor it might be a bit harder. I can't say I have every studied the ins and outs of reversing caps! :)