I need to know, how can you tell if you have a bad ECM or BCM?

I'm going to purchase a used car and the dealer is giving it to me for a lot cheaper than it is worth mainly because he believes it has a bad BCM, but when I called back the 2nd time, he said something about the ECM.

So, I was just curious as to what signs to look out for in order to determine which one in fast needs to be replaced, the ECM or the BCM.