How to determine whether the substrate of an LED chip is p or n type using the bench equipment?
Here is a typical LED showing the internal construction (from Wikipedia): 



The substrate is the side of the LED chip facing the "Anvil", in this case the cathode.  The cathode is always "N" type material. 

Some LEDs are the reverse of the above diagram, with the substrate being "P" type, as you can see from this photo taken from a Cree datasheet: 



The long leads are the anodes, and you can see each 'anvil' through the tinted epoxy.