daily riddle

The king dies and two men, the true heir and an impostor, both claim to be his long-lost son. Both fit the description of the rightful heir: About the right age, height, coloring and general appearance. Finally, one of the elders proposes a test to identify the true heir. One man agrees to the test while the other flatly re-fuses. The one who agreed is immediately sent on his way and the one who refused is correctly identified as the rightful heir. Can you figure out why?