After googling in net, i got same ethreal as Jason mentioned. And for Joe’s informaiton, Ethereal does identify RTP packet. But for that you should decode UDP packets as RTP. Then it will decode it to rtp (ex: verion 2, SSRC same etc..)
And for getting RTP stream u can run a Window’s net meeting and catch the session packets using ethreal and save as .cap file.
Using pcap calls u can retreive whatever the data or header information u want.