how can i know the result of "cap negotiation"

[宋喜佳]

Hi friends,
Maybe it is really a simple question, but I am afraid that i do not
now the answer. that is how can i know the final result of cap
negotiation. the caps of a pad tell us all of possible formats it can
support, but at actual running time, how can i know the format it is
using?
for example, we know the ffmpegcolorspace plugin support  many formats
at its srcpad, but at running time ,when linked to a sink element
,which supports many formats too, how can i know the final format they
use after cap negotiation ? ( I program with  C language)

thank you in advance!

Xijia