That's a bit trickier and may require listening to different packets. The level of difficulty depends much on the protocol and application being used.
As a starting place, check the tcpdump man pages. If you know the ports used by the protocol, the
port <port>,
src port <port>, and
dst port <port> primitives may be useful for narrowing it down.
Alternatively, you might find the
MacSniffer program useful if you are more comfortable with GUIs.