For many trans people, hearing the name they were called before transition can be a very painful experience. It can bring up memories of a time when they did not feel accepted for who they truly are. On the other hand, for some trans people, it may not be a problem.
To ensure trans people feel comfortable and welcome, do not ask about their birth names or even how their current name was chosen.
For more information on transgender people and the trans movement, check out the National Center for Transgender Equality.