Get an autopsy.
No, seriously, there is no ABSOLUTE. You can get calipers done by a trained person. You can get a bod pod done, if you have one in your area. You can do the underwater dunking if you have that available to you. You can get a dexa scan (expensieve). You can get a bioimpedence reading, though I find those pretty unreliable. Or, you can just go by your measurements and how you look.