I just had the alternator replaced, and the day after I picked it up from the shop, I am driving along and the coolant level light comes on. Sure enough, it is low, so I put more coolant in. The next day, same thing. Obviously, I have a leak, how do I diagnose this? There doesn't seem to be any leak under the car, so does that mean head gasket or block? How can you tell, and what is the typical cost to fix either one?