Changes in the color and viscosity may indicate that water has gotten into the engine's oil. If you're accustomed to checking your own oil level and quality, you may be able to spot a flood-damaged vehicle by performing an oil check.
Oil in a flood-affected auto will be a different color, sometimes compared to coffee with milk or a chocolate milkshake. The oil may be pale when it should be dark. Oil that's been affected by flooding may also feel sticky to the touch.
While you're under the hood, check the paper air filter, too. If it has water stains, that's another clue that water has seeped in where it shouldn't have.