Triggering flash

I have an analog camera that outputs a PAL signal. I'm capturing frames from
the camera using a capture card which is all working well. The problem I
have is getting a flash timed to the frame of the camera. Is there a way to
get some sort of sync signal from the camera and use that to trigger the
flash? What I was thinking was the PC could tell the flash to arm itself
through USB or serial and the flash could then be triggered by the sync
signal. I can do the serial or usb part and make a circuit but am not sure
how to get the signal from the camera.

Thanks for any replies,
Michael