Paul - , Aug 11, 2006; 10:13 a.m.
The first thing to do is measure the trigger voltage of the flash unit to make sure it doesn't exceed the 6v limit of the camera.
According to Chuck Westfall of Canon USA,
"Excessive trigger circuit voltage can damage the shutter units and X-contacts of most modern 35mm SLR cameras."
Just turn on the unmounted flash, and use a voltmeter across the side-contact and the center-bottom-contact.