[Tango-L] Copyright for tango music?

[dwyliu@gmail.com]

Hi All -- is there a good way to tell if a piece of tango music is
copyrighted?

For example, strictly speaking, if you play music at a milonga, film a
teaching clip, use music for a performance, etc., one should either:
1)  Have the permission of the person who owns the copyright
2)  Know and be able to prove that the music is now public domain.

Can anyone help with 1) or 2)  ?

For example, If we look at the "RCA Victor 100 Anos" Cds
- http://www.amazon.com/Rca-Victor-Ados-Carlos-Sarli/dp/B000267Z96
who do I write to, to ask permission to use the songs?

Alternative:
On the todotango.com website, they have a lot of music, much of which seems
like it might be public domain.  However, I wrote them, and they said that
not all their music was public domain.  How could I tell which was which?

Thank you Much,
David