A simple check a novice can perform; is to test the pressure in the cooling system. With the engine running - squeeze the top radiator hose with one hand, it should compress without much force, if on the other hand it feels rock hard then there is a problem with the cylinder head. (This is known as pressurising and is caused by a faulty head gasket or crack in the cylinder head, causing pressure from one or other of the cylinders to compress the cooling system... not good)
The vehicle may appear to drive and start as normal with this issue: but not for long.... so unless the price is right and the rest of the vehicle is is top condition - walk away!
At the very minimum one will need to change the head gasket, but more often than not there will be a small crack or warp in the head somewhere; so to avoid pointless work it is better to change the whole cylinder head, as new ones can be bought very reasonably nowadays.
If one is going to do the head work then you might as well replace the radiator, waterpump and timing belts at the same time, then maintain the engine as per recommended schedule with timely fluid and filter changes and you will have years of good service to come.