Estimating the wind speed is a valuable tool for kiting, primarily to ensure your safety and in doing so ensure that you are using the right tool (kite) for the job. This is based on what kites you own, your weight and level of experience.
The websites (and I usually check all three as their forecasts can vary) I use for forecasting the wind and making that decision whether I load up the car are...
It is also essential to know your kites wind range, your limitations – in and out of the buggy and if in doubt get out!
This is a good guide to estimating the wind speed on the ground
I always look at the trees to see how much they are moving, when I started to fly kites again somebody once said "welcome to tree watchers anonymous!" and that is so true!