The rib test is a good guide, but there are definately other ways as well.
When I got Miss Kitty she was 3 years old, a stray, and hadn't been fixed. She had many loads of kittens before I got her, so her belly and nipples were already streched - not fat, but streched.
Coupled together (being fixed, going from a stray to an indoor home cat) Miss Kitty put on close to 4 LBs the first year - going from about 9 to just under 13. I was also using hairball reducing food, which has more fats and oils and is obviously more fattening to kitties.
After this visit I noticed when looking at her from above she was a bit plump in the middle, so I put her on a diet of sorts. I changed her food, and opened up the basement for her to run around and play in. She has shed the extra bulge and now isn't skinny, but isn't overweight - I'd say she's about 12 pounds now, which we both feel good about.
PS - This is kind of a funny post-script...my friends thought I was loony because I put up a feeding chart for Miss Kitty (I work odd hours and I wanted to ensure I didn't over or under feed her), and...I went to that Fat Katz website, and found the biggest, fattest grey tiger kitty picture I could find. Then I pritned it out huge, on four sheets of paper, made a poster...and put it right above Miss Kitty's food dish. I wrote, "Miss Kitty - do you want to look like this??" on it. >:-) I thought it was quite amusing. My friends think I'm batty.