I'd do as david sugests...compress the tensioner in a vice protected with soft jaws, insert a pin (a small allen key works fine), then re-install the tensioner and remove the pin. You should now remove the covers from the front of the cam sprockets to asses the position of the cams. I'd be very careful not to run the motor until you do as there is a chance it will have jumped a tooth. Theres also a chance it hasnt if your lucky...but better safe than sorry in my book.
Plenty of links on this site with pics of changing the cam belt.
Elliot