Possibly a retarded question, but what can be counted on in the present or future to make sure a real human is making a valid order?
You can fake some kinds of physical cash. The dawn of supercomputing may make 16 digit cards (plus 3 cvv) obsolete. There are some current issues with bots making purchases for high demand items such as concert tickets. Obviously there will be problems that come from some biometric data, too (3d printed thumbs, anyone? Or 'member Gattaca? Or what happens if your data changes because of some injury?). Stuff's kinda messed up seemingly any way you go.