How to know if arduino activated its short breaker?
It sounds like connecting your Laser to the 5 volts power supply did draw too much current, thereby the 5 volts dropped to very low voltage.  

The Arduino didn't "protect" itself.  The Arduino simply shut down because of voltage being too low. Then, upon reapplication of 5 volts the Arduino re-started as normal. 

You don't mention any details about the Laser.  But if your Laser is simply a Diode, the Laser diode would draw a lot of current, resulting in the 5 volts power source giving reduced voltage.  

If your Laser is a "laser module" with a built in driver, perhaps you connected the Laser backwards to the 5 volts.  

EDIT 11/20/2015 :  IF you are using the Arduino UNO ,  : The  PolyFuse only protects from 500 mA overload on the USB connector.  Schematic for the UNO shows that 5 volts for USB is called "XUSB". The  PolyFuse connects from "USB VCC" to the XUSB.  When you connected your Laser to the +5 and ground, the  PolyFuse was not involved. (Unless of course you did connect to the USB 5 volts (XUSB), and didn't tell us that.  

HOWEVER,  if you powered your UNO from the USB port, then the  PolyFuse WAS involved and did shut you down as others have commented.