How do I tell if my snake is too thin/needs MORE feedings?

The guy I got it from (not a snake guy, was watching the snake for a friend and the friend "abandoned" snake) told me to feed one medium sized rat every 2 weeks. We got him Monday (Sept. 22) said he was fed the day before. Some people told me I should really be feeding every week (at least) so on Sunday (Sept. 28) we fed FT medium rat. Snake gobbled it up! So cute in doing so too... as the last part was going down, snake turned so my son could see real good... seemed like snake was saying..."see I am good snake and eating ALL of my food"... I guess you had to be there BUT **I** thought it was real cute! It just kind of seems like it is hungry and I want to make sure it is getting enough to eat (NOTHING in my house ever goes hungry... we are ALL well fed....)

Was told the snake is 15 years old.
Measures 4' 11"
Weighs 4 pounds 14 3/4 oz. (2234g.)

As soon as my batteries charge, I will add some pictures.
Any help/advice would be appreciated.