Right click on a packet and select "Decode as...". Then choose "RTP" in the list of "Transport" protocols.
If you want to decode some custom-made protocol, you'll have to get or compile a dissector, which can be quite troublesome... Fortunately, PCAP format is simple and plugin directly to it is easy (I happened to just have made one last week, I might make it Open Source if my boss agrees).
Thanks to @GuyHarris who stated that automatic RTP decoding is performed when SDP packets are detected that initiate the RTP session. There also is an option in the "Edit > Preferences" menu, choosing "RTP" under the "Protocols" tree: "Try to decode RTP outside of conversations".