1988 Pontiac Grand Prix 2.8L V6.

Replace Serpentine belt and power steering pump
Question = Belt broke on the freeway, car overheated (Daughter driving). I replaced the belt and added water to the radiator. Car starts fine and will idle but as soon as you give it gas it stalls. LOTS of white smoke.

After I run it for a couple of minutes I have to add water to the radiator. I am thinking head gasket or cracked head? what else could it be and how do I check it to find the problem?

The car was most likely overheated pretty badly to be running like it is now. When you have white smoke out the exhaust, that usually means coolant is being combusted in the cylinders along with the fuel and air. With an extreme overheat issue, cylinder heads end up warping to the point that the head gasket cannot seal the gap and lets coolant into places it shouldn't be. As you also figured, there could be a cracked cylinder head, and possibly even a block.

It may not be that bad, but you definitely at least warped the heads or blew out the gaskets from the heat. You could do a quick check with a cooling system pressure tester to see if the system holds pressure, which it probably won't. You could take the spark plugs out and check for coolant in the cylinders as well.