[gmtech1]

I would rent/buy or borrow a cooling system pressure tester. Look closely around the intake gaskets, especially under the throttle body area where the intake meets the block. Also, check under the right side of the dash for damp carpet or any other traces of coolant that may indicate a heater core leak. There are also testers that will check for Hydrocarbons (exhaust gases) in the coolant to help verify a head gasket leak.