You can't tell. BMW suggest 40-60k. I did not know the last time mine was done
(171k on it) and took a big chance for 5-6k. When I changed it the belt had cracks
between every tooth, it was ready to break. If that happens stuff breaks!!! Just
do it if the age is unknown. It is not too hard to do. If you are going to try I'll give
you a few tips. BMW wanted $1000 to do it and that was out of the question. Of
course you should do the water pump at the same time. Mine is an '86 325.