I might be wrong, but judging from the leaves in the background and the branch it is sitting on which I think is part of the same tree, it's one of the tree species (I have no idea what tree species it is, but it is rather characteristic, very open, usually quite horizontal branches) these birds tend to favor to rest on or hunt(?) from. I guess it offers them a great view.
BTW Not sure if not being able to identify an eagle shows a lack of knowledge of birds.
I've too found it very hard, especially because birds of prey tend to be different in seasons/ life stadium/ localities. And some species are very much alike.
Though, this species, indeed the Crested Serpent Eagle, in adult form is one of the species I confidently dare to identify as such. It's probably the most common bird of prey I've seen in Khao Yai. I've seen a few juveniles which, even though they shared some characteristics of the adults, were quite different and could make identification harder. The yellow feather-less part near the bill is a characteristic that I haven't seen in other birds of prey in the area.
In flight, seen from below, the adults have a dark band at the outer edge of the wing followed by a broad light band. And same on the bottom side of the tail. This pattern, as far as I know, does not occur in other birds of prey in the country, so that makes identification when in flight quite easy.
Here a pic that shows this banding.
The pattern on the feathers on the belly is very characteristic as well. I can't find a picture in my collection showing it, though.
It's a beautiful bird, but I love snakes so I have mixed feelings about it ;).