Go by the factory manual that came with the car, for the 1999 Esteem this basically means that you need to change it every 60k miles under normal driving conditions for both the 1.6 and 1.8 engines.
There really are no symptoms for a timing belt that needs to be changed, the timing belt stays a timing belt until it breaks and then it becomes a piece of shredded rubber, at the same time your pistons and valves get the kiss of death and slam into each other.
There is no available manual for the Suzuki Esteem aside from buying a factory manual from the dealer, I'm not sure why neither Haynes and/or Chilton made a manual for this car.
But anyway there is hope, I have a growing website at:
http://home.comcast.net/~lj_robins/99suzuki/ click on the
BELTS section to view how to change the timing belt, its really not that hard.