how should i know after UDP RTP is there

hello every body I have writte a code that strips off all the headers and extract the payload part of the packet and then dumps in to another file and then plays it using a sox system call.this is an offline process,I have been given some TCPdump files but how do i make sure that after UDP header RTP will follow it.I hope every body got my point,
plz reply to his query I am awaiting for the ideas.

Thank you.