xLODGen picks up only TREE, STAT or ACTI records with "Has Tree LOD" flag set, then it will print out found and not found billboard names, so you can simply copy it from messages log.
However I don't recommend you to use Tree LOD system in Fallout 3 and New Vegas, it is bugged there. Sometimes when approaching a tree it's LOD will disappear and then the full tree reappear a second later. The other bug is when LOD doesn't switch off at all up close for trees placed by plugins. That's why I use it only for smaller objects like bushes where this is not that apparent and distracting in game. New Vegas doesn't use Tree LOD system at all because of those issues, Obsidian guys were clever enough to avoid it.
For real trees I recommend to use Objects LOD and create _lod.nif mesh, check optional FO3LODGen resources for your mods on Nexus. It is a flat mesh (I stole it from New Vegas yucca trees LOD) with billboard image on it and ~15-20% empty border to avoid aliasing issues of LOD atlas texture (black lines at the top of distant objects, you can see them even in vanilla FO3 game sometimes).
Ideally those _lod.nif meshes for trees should be real low poly meshes if you have time to model them for better look and to avoid aliasing bug. They can have any number of shapes using different textures, however all textures must be not tiled with UV values in 0..1 range. Vertex colors are not supported by the game in LOD. Also records which use _lod.nif meshes must have "Visible when distant" flag set.