If a file A is not symbolic link, we can use describe to get the labels which were applied on its current version.
But if it is symbolic link, we can just use describe to get a relative path of the target file. Is there any easy way to list the labels of the target of a symbolic link?
No, for the same reason you cannot access their extended pathname:
Symbolic links are not versioned and, hence, do not have version-extended pathnames.
Symlinks are not catalogued in the version history of the element. They serve only as a pointer to a target version, and allow you to create multiple access points to a single version.
You must first resolve the link element destination in your current view configuration before listing the label on that destination element (file or directory).