I have a spot that I like to take sunset pictures. I find that the most spectacular moments last for only maybe 5-10 minutes. What I would like to do is know about a half hour before that a sunset is going to be spectacular, so I can grab my camera, tripod, walk 5-10 minutes to my sunset spot, set up, and have a few minutes to spare in case something happens. However, I've often missed really cool sunsets, because I just don't know when to photograph them. How can you tell beforehand when a sunset is going to be spectacular? Thanks!
In my experience, if there's lots of cloud overhead, but a break in the cloud towards the western horizon, then as the sun dips below it will illuminate pretty much the whole sky. Don't forget that the best lighting occurs AFTER the sun has actually set. Make sure you have some foreground interest too, if possible, just to add a bit of depth.
One of the things I've learnt about shooting sunsets is do not try to guess when a good one is going to come or not ... if you've decided to go and shoot the setting sun then go and don't let the weather or other conditions deter you.
A good example of this is one day I was on the way to a chosen spot for a sunset session and a storm rolled in obliterating the light from the sky, I almost turned round and went home but decided if nothing else this would be good practice. As I was setting up the storm opened a hole in the clouds and the light coming through refracted off the moisture and the resulting image is my most popular one to date! http://www.thetrueshot.com/play-ball/
Some things you can look for to increase the chances of a good shot are clouds above the horizon that will catch the light on their edges just after the sun sets, also often the best light is 5 to 20 minutes after the sun sets so don't rush off just after the sun goes down.
The most important thing is simply being at the right place at the right time, the more times you shoot the sunset the more chance there is of having the camera setup with a spectacular light show in front of you.
Good luck with your favourite spot!
You'd most likely be looking for a day with clearly-defined clouds (any amount is fine) that are high in the air, this way, when the sun moves underneath the cloud layer, you can photograph it. If you're lucky, you may even get some shots of sunrays poking through the clouds, as well. ;)
I think the more important aspect about this is not the weather, but the location. Bodies of water are spectacular for this type of shot, as are high points of ground (like the tops of large hills and small mountains) that overlook a lot of flat land and/or water.
So, in a nutshell, I'd worry less about the weather and more about where, specifically, you want to take this picture. :)
If you are looking to capture one of those truly superb sunsets that you see from many great photographers, the trick is often to simply wait for it. Great sunsets don't happen that often, and when they do, you may only have as little as 30 seconds to capture that single magic moment before it is gone. One of the tricks of many great landscape photographers is vision and presence of mind: Being able to visualize ahead of time what a particular vista might look like with that perfect sunset (or sunrise) lighting, and having the presence of mind to BE THERE when it happens.
An excellent book that discusses just this topic is "Photography Essentials: Waiting for the Light" by David Noton. David explains a few times in this book how he would scout out vistas, envision how they might look with the right kind of lighting, and wait for a days, even a week, to be there when that lighting occurred. A lot of landscape photography is seeing the potential in a landscape when it is dull and lifeless. The rest is just being in the right place at the right time to capture what you envisioned. Since most good lighting occurs around sunrise or sunset, you can imagine how most of a landscape photographers life is spent chasing the perfect light.
So, to provide a simple answer to your original question: You don't "tell"...you "wait", with expectation, and capture the lighting you EXPECT.