It all depends on what the engine speed is when the belt snaps. If your just moving away you might get away with it. If your caning it then it will probably knacker it in style.
The moral of the story is to change the belt if unsure of the history, if you've been told its been changed but have no proof in the history then change it again as its cheaper than a rebuild and new valves.
Also always use ORIGINAL PARTS for it and always, always, always fit a new cam belt tensioner as this is normally what seizes up and knackers the belt.