My wife's car has been having an overheating problem for some time now. There are no noticeable leaks in the coolant system, however, about once a week I have to put at least a half gallon of coolant back into the system because it keeps getting low and the car begins to overheat. I have noticed though that the overflow is completely full but the radiator itself keeps getting low. I took it to a mechanic and after testing it, he said that it could be a cracked head, but he wasn't sure. I know that it could be a cracked head but I need to be sure before I figure out my next course of action. The secondary fan works fine as does the thermostat, and I am not getting any white smoke out of the tailpipe at startup (I heard this is a symptom of a cracked head), is there anything else I should check?