One test is a cooling system pressure test. Put a gauge on the radiator cap and run the engine and see if the pressure rises and falls with RPM and/or pump up the cooling system and see if it slowly leaks down - might be a leak only under pressure.

2nd test is to pick up a chemical test kit for the coolant to detect the presence of exhaust gas contamination. NAPA sells a kit for around $20 as I recall.

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