[VOIPSEC] RTP or SRTP inside UDP - how understand?

[Sergey Vointsev]

Greetings gentlemen!

I've got a question, but Mr.Google keeps silence on it. So I think
this maillist is the best place to ask.
How can we determine if the packet we have is RTP or SRTP packet, i.e.
payload is encrypted or just encoded? As I understand, they have
absolutely the same header. The only difference is encryption and
optional fields MKI and AUTH_TAG. Moreover, UDP hasn't got any fields
for indicating higher lever protocol type.
Could you please explain this moment?

Thanks. Sergey.