Used this once to debug an issue where it turned out unwanted exports in one library A where used to satisfy imports in another library B and thus preventing library A from being unloaded. Fun times.
The ldd command is also essentially just a wrapper script that sets LD_* variables that get the dynamic loader to do all the real work.
The man page ld.so has ENVIRONMENT section that looks pretty good.
Otherwise one of the most reliable options is to simply grep the source code for calls to getenv, I would not be that surprised if their are some extra ones that are undocumented.
The ldd command is also essentially just a wrapper script that sets LD_* variables that get the dynamic loader to do all the real work.