Help reversing Flicky on Megadrive game

Hi,

I am looking for some advice and/or help

I want to reverse a specific area of Flicky for Megadrive, but I have no real idea where to start. I come from a Computer Science background and am familiar with the process of debugging. But my Megadrive specific knowledge is a bit lacking. I've read vext01's slides on reversing which have given me a little more insight. But I still need pointing in the right direction.

So the question I want to answer using debugging is:
What logic governs whether or not a cat will drop a diamond when it disappears?

You can see what I mean in this video:
http://www.youtube.com/watch?v=9_jcUGPURt8&t=29s

If you watch some of the rest of the video, you'll see that when cats are knocked out they do not drop diamonds every time.

But where to start looking?

First I thought about watching for the diamond sprite, but that's displayed after the logic that governs the diamond drop.
So, thinking a bit more I remembered that after the cat disappears there's a puff of smoke. After that the diamond appears (or not, according to the logic).

So I need to put a break/watch on when this graphic is accessed. But first I need to find it in memory.

Then I'll need to trace through the code that executes from this point on, for bunch frames.

Any thoughts, advice or additional help appreciated.

Thanks