'sh ip arp' should give you want you want... Or you could cheat and use a network analyzer, compare the mac addresses. Some of them even identify the manufacturer by mac. I had a free one on my iPhone but can't remember what it was called.

"There is one thing and only one thing in which it is granted to you to be free in life, all else being beyond your power: that is to recognize and profess the truth." - Leo Tolstoy