How do I know when I'm fat?

Let me first say I am not a person who is ridiculously self conscious and Unrealistic. I am very confident about my looks. With that said here is my question... So most my relatives not including my parents are overweight. And I have a bigger bone structure. But I'm also short so I have o be really athletic to keep my self fit. Anyways over the summer I haven't been exercising as much because it's to hot and I'm a cross-country runner meaning I had to be outside. And in addition I'm not doing a school sport this fall because it interfered with other things. Anyways I've noticed that when I sit down or slouch my stomah sticks out more than usual. When I'm standing up normal I look fine but otherwise I look like I've gaine weight. Witch I know I have. So my question is will I know when I become fat? If not how do I find out.
How do I know when I'm fat?
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