It turns out you can define it from looking at radioactive decay (or any other interaction governed by the weak force). Feynman famously had a short description that ended with the punch line
> Suppose, after lots of communication you finally can go into space and meet your alien counterpart. If, as you approach one another, the alien extends its left hand to shake, watch out! He’s made of antimatter!
because even the physics of weak decays of will invert in an antimatter universe.
> Suppose, after lots of communication you finally can go into space and meet your alien counterpart. If, as you approach one another, the alien extends its left hand to shake, watch out! He’s made of antimatter!
because even the physics of weak decays of will invert in an antimatter universe.