The way to tell when a cat is carrying more weight than they should is that when you feel over her ribs you should be able to make out the individual ribs but also feel a fat layer on top. From a bird's eye view, you should be able to make out a waist in between where their abdomen ends and their back legs begin.

Is she a breed, or just a moggy? I have a siamese and they are naturally slim, so even a little pudginess shows on him!

I would recommend reducing the amount you feed - if she is still on kitten food just feed a little less. Kitten food is really high in calories because they have a lot of energy to burn what with growing and playing. If she isn't very active she won't be burning those calories. I feed my two cats 1/4 of a cup of cat biscuits PER DAY. They are happy and fine on that amount (1/8th in the morning, 1/8th in the evening).
It may pay to switch to an adult formula food (less calories), or promote more exercise - play with her more and get her active - chasing toys/string/birds is a fun way of exercising.

Good luck with things, let us know what happens!