The simple answer is that if a noun doesn't have الـ at the beginning it usually ends with -un ٌ / -an ً / -in ٍ
If it starts with الـ then there isn't any N sound at the end. In this case it ends with -u ُ / -a َ / -i ِ
There are exceptions to the rule ofcourse.
Also you don't say / read the case-endings (-u(n) / -a(n) / -i(n)) for the last word you say / read.