1999 Ford Explorer

I recently purchased this vehicle from a private owner, who informed me that she is selling the vehicle after 11 years of ownership. She has already informed me that the vehicle overheats after driving for a short time. Upon test driving, I allowed the engine to idle for 15 minutes and drove for about 7 miles before it began to overheat (of course, I shut off the engine before the gauge completely reached the "H"). I did make these observations while the engine was running: 1. There was no smoke or excessive water coming from the exhaust. 2. The oil appeared to be normal in color and consistency (no coolant mixed with oil) 3. Engine did not skip or stall while idling or driving 4. There is coolant leaking from (seems like) the bottom front passenger side of the engine (maybe under the alternator, but it seems to only leak while driving. The previous owner also informed me that she recently replaced the radiator and that a mechanic told her she may possibly have a cracked head. Any suggestions of the cause of overheating or where to look for the leak before I replace the water pump or intake gaskets? Thanks!