how can you tell a cats age

hey folks, 

just wondering how I could find out a cats age. He isnt mine. he is owned by someone but I dont know who. but this cat has come around my house for the last 3 years. I feed him soft food once a day. He has a smooth bald spot on his back and I just wonder how old he is. he is a well fed cat. 

It's pretty hard to tell how old adult cats are for the most part.

The condition of their teeth sometimes is helpful since a lot of older cats wind up with diseased gums/teeth. It's not as common in younger cats.  Sometimes, much older cats start to loose fat in the muzzle and hind legs so that's another way to give an approximate age.  They also sometimes start going a bit gray as do humans.  There are a few other minor changes in coat quality and appearance.

Our rescue was averaged to be between 3-5 years old when we got her.  They could not pin point a definitive age.  It's much easier to do with very young cats since they change so dramatically from week to week.