It's straight forward with an SLR viewfinder. The sky goes darkest when the polariser has maximum effect. When the viewfinder compensates, you have to note how the brightness of the sky varies relative to the other subject matter. Usually you want to minimise the brightness of the sky relative to most of the other subject matter (other than awkwardly reflective surfaces). As already mentioned, these effects are most evident when the camera is pointed at right angles to the horizontal direction of the sun on a clear day.