Assuming you have a camera capable of it... Manual mode, spot meter off any clouds' whitest portion, expose at 2 to 3 stops more exposure. Use shutter speed, so if the meter reads 1/500, expose at 1/125 - 1/60. If no clouds are present, meter at where the sky registers medium blue, expose without compensation (the way that the meter reads). If you aren't familiar with the latitude of your camera, be sure to check the image in camera.
(optional technical stuff)
What's happening here is that when you spot meter off a specific tone the meter is calibrated to render that tone as middle tone, i.e. not black shadows, not white hilights, but close to the middle in between. When you increase the exposure, that tone is rendered lighter than what the camera expects. So when you meter off the whitest portion of the clouds and provide more exposure than what the camera suggests, the cloud becomes lighter in the image and the rest of the scene follows. If you didn't adjust, then the whole image, cloud and all, would be under exposed.
In the second scenario, because the sky is already "middle blue" the spot meter will provide accurate exposure. You could carefully meter off or near the sun, however because the sun is SO bright it will throw off the entire exposure since the camera only has so much "latitude" - the range from darkest to lightest that it can effectively resolve.