Well you can do a body test.
Look down at your cat.
You should be able to see a waist when you look down from the top or where you run your hands from its ribs to its hips.
Run your hand along its abdomen from it ribs to its pelvis and it should be indented.
If you put your hands on the side of its chest you should be able to feel its ribs without a thick layer of fat over them.