What to look for when getting used aluminum heads?

[bjl95mustang]

I've found some gt-40y heads for $600 with 4000 miles on them and some edelbrock heads for $700. I'm going to go look at the edlebrock heads this week and I was wondering what to look for. The guy said that he would disassemble the heads so I could look at the valves and ect.

Thanks.