I guess we would know because animals don't tend to have much more bone than they need. You should be able to estimate the mass of an animal based on the size and structure of it's leg bones (and whether you think it lives it's whole life in water). Then you assume it has enough muscle to move that mass, which gives you an estimate of the density (and locations) of some of the remainder. Split the rest between organs, fat and more muscle in proportions that seem reasonable based on existing animals.

Then discover you're still surprisingly wrong, probably. But it should rule out the pudgy T-Rex at least.