Cloud bases can be quickly estimated with this formula:

Temp (F) - Dew Point (F) / 4.4 x 1000

For example, if the temperature at the surface is 65 deg F and the dew point is 40 deg F, the cloud bases should be at about 5,600 feet agl (65 - 40 = 25/4.4 = 5.68 x 1,000 = 5,680).