2003 Chevy Impala Milky Coolant Fluid

We are thinking of purchasing this used Impala but in checking the vehicle out we noticed a light brown milky and foamy substance on the radiator cap and down inside the radiator. We think it may be coolant mixing with oil but we aren't sure. Is there something we can check to make sure this is not a major problem before making an offer on this vehicle. We did take it out for a test drive the the engine did not get overheated.