Specialists claim that water leaches grease from bearings. As a result, you should hear a rumble from the work of mounted units: generator, air conditioning compressor. But not every ear can catch such subtle things. Therefore, at least make sure that the engine starts half a turn.
You can pull out the dipstick and look at the oil. If it is fresh and transparent - there is reason to think, why would the seller decide to be generous? New filters - air and oil - should also alert.