Possible arduino burned (5V shorted to Vin), explanations and suggestions
If you have another Uno then your best bet is either:

1) Swap the suspect processor into the known good board, test it. 

2) to buy a replacement chip (ATMega 328-PU), use this link to flash it, if it works you win, if not you can at least use the new processor again.

I have done this myself &amp; it worked well, it isn't easy to be sure any other way as the chip needn't fail in a predictable manner. The chances are it will take a large current if it's dead, until it burns out &amp; goes open circuit. Once the magic smoke has come out you know it's dead... the Arduino power supply is reasonably capable of looking after itself so your chances are good.