1. Check surface flatness yourself. Most A12 do not need head skimming (head milling). Use a straightedge and a .004 inch feeler gauge. It is allowed to be up to .004 from flat but I've never even seen that much even after overheating and blown gaskets. Normal is .002 per the Datsun factory service manual.

2. carry a water bottle. Place the head upside down and fill the combustion chamber with water. If it holds for 5 minutes, the valves are sealing good.

3. check for play on a valve stem. Use a small compressor (I paid $10 for one at a flea market) and see if the valve is tight side-to-side. If it's not, go on to the next head. If it is tight, check a few more (or all of them). Visually inspect for gauges in the head face and manifold faces. Clean-looking heads are best. Pass on slightly rusty heads. Wiggle the valves to see if the guides are OK (this is the part most often wearing out on A12). There should be almost no discernable wiggle with the valve slightly off the seat. If there is, it requires new guides ($$$).