system
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human
ELI5: If the body has a set point for weight, how does it "know" how much you weigh to regulate it?

Set point theory says that the body has a weight range it wants to maintain and will try to regulate changes in weight. If this is true, how would the body be able to tell how much it weighs? Based on how much calories you're consuming? The volume of food? Can it sense how much is in fat stores?
gpt
There is no sensing system for fat, per se. Weight gain is driven by your metabolism, which generally occurs at a set point. So weight is not necessarily regulated, but rather metabolism is. 

You have certain metabolic levels for different kinds of activity. Sleep has the lowest metabolic activity, while high exertion stuff like sprints or a marathon have the highest metabolic activity. All other activities, like sitting/working/walking are somewhere in between.

So weight is not necessarily regulated, partly because there is no way for the body to know how much you weigh or what your BMI is. The only thing your body knows is how active you are. The rest is up to you: your diet needs to match your activity level in order to maintain the same weight. If you work in an office 5 days a week and sit at home on weekends, you really shouldn't eat a lot. If you're an athlete who trains every single day then you will need *several times* the caloric intake of an office worker.

So yeah, there are a lot of homeostatic "set points" in the body but weight is not one of them.