Pretty basically, unless you buy the cylinder head from a cylinder head builder such as buying a Sidewinder from Todd Marsh, you should look at the valve guide clearance. All sorts of aftermarket heads have come out of the box with too tight a valve stem clearance.
Once you have the heads disassembled it is a simple matter to check that the valves are seating and that the valve job is optimized. Again if an engine builder assembles your heads you are most likely OK. But, I have seen factory OEM cylinder heads with absolutely horrible valve jobs. Not that they didn't seal. I was shocked to see a one-angle valve job with no effort made to blend either side.

There are probably just as many cylinder heads out there that came out of the box and were bolted on without any additional prep work. So it just boils down to, "Do ya feel lucky, punk? Do ya?"

R.