How to find out true names?

Question

Using the spell planar binding I can bind a individual by calling its true name. But how can I find out a true name besides the discovery True Name. I guess I cannot find true names in books.

Answer 1

I guess I cannot find true names in books.

Yes. Yes, you can.

With a lot of research.

If you lookup the rules for Binding Outsiders (from Ultimate Magic), we will see that you can research the true name of an outsider on old libraries. This a long and tricky job, because strong outsiders do not want their true name to be known, and thus they have many names that they are known for, as to mislead those attempting to obtain command over them.

To discover a single outsider's true name, a spellcaster must spend at least a month in a library or on a quest of discovery to uncover occult mysteries and riddles hidden in the pages of books, scrolls, and glyphs written millennia ago, buried in ancient temples or found among the ravings of madmen's spellbooks.

At the end of this month, the GM makes a Knowledge (planes) check for the character. The DC is 10 + the creature's Hit Dice. The GM can increase the DC by +2, +5, or even +10, based on the power of the outsider or the circumstances of the true name search. A failure by 5 or more turns up false information that may expose researchers to unexpected dangers.

Each of the Book of the Damned campaign setting books (3 volumes total) also brings the true names of a few evil outsiders, along with things you can do to discover their names and have an easier time attempting to bargain with them.

Some of those tips, for generic outsiders, can be found on the SRD pages about Binding Demons and Binding Devils, for instance.