The first symptom of Lethal Yellowing is that all the blooms turn black and all the nuts large and small fall off. After that its already too late. The top growth sprout turns yellow and can be seen from a distance. Whole tree does not necessarily look yellow, just the top. After that the top breaks off and you are left with a toothpick.
More common is the palm beetle ., The bottom leaves droop and start to turn yellow before it dies. Lethal Yellowing is very distinctive in its symptoms and easy to diagnose. First look at the blooms, if they are black and it drops all the fruit its L.Y. if not black and still has fruit its the beetles.