There are some areas of a car that won't dry no matter how hard someone tries to cover up flood damage. Look closely at all lamps. Headlights and taillights may appear foggy when water has accumulated inside.
The same can be true of the instrument panel, and interior and exterior mirrors. If they appear foggy, they, too, may have moisture that hasn't evaporated.
Water also tends to pool in the overhang inside the wheel well, so check for signs of dampness, corrosion or tell-tale water lines. Examine the doors and trunk for any water lines as well; they indicate how high the water level may have been when the car was flooded.