asterisk users - Oct 2004 - Determining that call was transferred

Hi,

I wish to determine whether a call is original call or it was transferred by
someone.
I need to do it within AGI script.
Can I consider the following statements true: 
1. if agi_extension != agi_dnid then the call is a transferred call
2. the call was transferred by extension agi_extension

My experiments seem to confirm it, but I'm not sure it's true in 100% of
the cases.
Thanks,

Michael