Car Overheating,no Visual Leaks,What To Do?

First suspect would be the thermostat, especially if there is no heat from the heater.  Second would be too high a ratio of antifreeze in the radiator.  That's the only guesses I can think of.  The thermostat can be tested easily by removing it and putting it in a pan of water and heating it.  If it is totally bad, it prob won't open at all but to check accurately you need a thermometer and you need to know what temp it is supposed to open at.  The temp may be stamped right on the thermostat.  If not, a parts store may give you that info.