asterisk users - Feb 2013 - Transfer by transfer button, distinguish in dialplan

Hi list!

We're using Asterisk in a setup where you can transfer a call via the 
Asterisk feature, or via the Transfer button on a SIP phone. Both work.

However, in my dialplan I cannot distinguish normal calls from calls made 
by pressing the Transfer button on a phone.

To clarify, I would like to know the difference in the call from A to B, in 
these two situations:

1 (normal call)
A calls B

2 (transferred call)
C calls A
A presses "Transfer" button and calls B

Does anyone know a way how to accomplish this? Is it at all possible?

Thanks in advance for any hints,

Roel