What to look for on checking 302 Heads

Since my slant is down for some machine work, I started back on my 273.
I picked up some 302 castings a few years ago.
I remember the seller telling me they were not cracked.
But they are missing a couple of valves. Why I will find out.
I am going to disassemble them and clean them.
Where should I be looking for the cracks that I have been told they develop?

Now if I can just find that pan I thought I had.........