Clouds: Usually there are more interesting skies when there are some clouds at sunset. But usually there are fewer clouds towards sunset. Cumulus clouds generally start forming in late morning, peak in development by mid-afternoon, and then start to dissipate as sunset approaches. High cirrus or stratus clouds stick around longer and they make great colorful "streaks" across the sky. They reflect the colors and make the sky more interesting and colorful.

As far as predicting color brilliance itself, it usually has a lot to do with particles in the atmosphere. If you want a lot of pink and crisp color lines in the clouds, you will need an atmosphere that is clear with low humidity. If you want the sun to be a hazy, large ball with the brightness a bit "tamed", you need more particles in the air (pollution) and/or high humidity.

Unfortunately there are no easy answers in determining which sunset will be the best. The best way is to learn for yourself and just get out there consistently to witness it yourself. You start to pick up on clues after awhile.

But as someone who has taken many sunrise and sunset photos, I can tell you that every sunset is a surprise. Sometimes the one you think is a dud turns out to be brilliant at the VERY last second. Sometimes when you start out to set up and see all the clouds, you think it won't even be visible. Then, all of the sudden, it appears at just the right moment and you are eternally grateful you stuck it out!