You can do a chemical test, testing the antifreeze to see if combustion chamber gases have been mixed in. Check the oil for a milky appearance, especially on the dipstick right at the top of the oil line or on the bootom of oil filler cap. Another way is a cooling sysytem pressure check, escpecially if the leak is outside.

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1983 380SL
1995 C280
1995 S500 coupe
1990 Mazda Miata
2004 Suzuki Hayabusa