on the dipstick of there are beads of water on it, or in the coolant expansion tank if there is a mayonaise type residue, then the head has gone and needs replacing.
How do you work that out? that the very worst diagnosis.
The head gasket could just be blown, even if the head is warped, it can be skimmed.
More likely as the geezer said, he uses it for short journeys, so its probably just condensation.
Take it for a really good thrash down the motorway, see if it clears up.