How do you know when to change timing belt?

[Martin W. N. Suryadarma]

Hi all, 

I bought my 89 Audi 100 last year. It had 98k miles then, it has 108k
now. How do I know when to change the timing belt? Or whether it has been
changed or not? The previous owner only had it for 1 month because his
wife hated it, and he didn't get any service records from whoever he
bought it from. I just want to make sure so that the car doesn't die on
me. Thanks. I love the car, BTW. 

Martin