When buying Vortec heads from junkyards (or any head for that matter), the chances increase that the heads will be cracked. There are ways to tip the odds in your favor, though.
To find heads from an assembled engine that have a better than average chance of being uncracked, look for:
heads from a wrecked vehicle. These stand an excellent chance of being good, as long as the other things listed here pass inspection.
engines that do not have the thermostat removed. A missing thermostat is an indication the engine was running hot.
engines that still have antifreeze in them. Engines with straight water, or without any coolant showing, may well have been losing coolant. No one replaces leaking coolant with antifreeze- they will use straight water "until I fix it". By then, it's often too late.
engines with water in the oil or oil in the water. This is a sure sign of a blown head gasket or cracked casting, either of which can lead to overheating and cracked heads.
vehicles that have antifreeze receipts or antifreeze or water jugs laying around inside the vehicle. That's another sure sign there was a problem with the cooling system.
Look for heads having a spark plug (or spark plugs from adjacent cylinders) with no unusually colored deposits- or a lack of deposits that were removed by coolant getting into the chamber(s). Head gaskets leaking or a cracked casting that lets coolant into the combustion chamber will make the plugs look a lot different than a normal plug- look for uniformity.
Look for engines that do not have evidence of stop leak in the radiator/cooling system. If it has stop leak, it was leaking. If it was leaking there's too big of a chance it overheated and cracked the heads.
Signs of stop leak use is a sludgy or copper/silver colored metallic accumulation inside the radiator fill neck, the same thing under the radiator cap, and possibly even in the overflow tank.