1st thing I do is to look around the seller's area to try to get an idea of whether he's got his act together...
As far as the heads, look for stripped threads, sunken valves, evidence of porosity and anything that looks cobbed up.
Valve & spring seating surfaces will be hard to judge if valves & springs are installed.
Mis-matched valves, springs, etc would worry me.
You'll need to know if they've been surfaced or cut and by what amount, whether intake face was milled if much was cut off the deck.
Like any other heads, what springs, retainers, guides, and whether that works for your setup.
Not sure if anyone's making Aluminum heads without accessory provisions, not a problem with Eddys or Stealths or Indy EZ1s.
Some grinding at one end of a BB head that interfered with a distributor does happen depending on what they guy's combo was, but like all things, should be nicely done.
And I would hope for documentation, but some guys just don't keep the paperwork.