The easiest is to tcpdump the traffic (Or ssldump if it's encrypted).
On the bigip it'd be something like
tcpdump -i 0.0 -nn -e -s0 -X host BACKEND_IP_ADDR and port BACKEND_PORT
-i == specie the interface. 0.0 will snoop all interfaces
-nn == don't translate ip's or ports to names
-e == print the link level header when dumping the traffic
-s0 == capture all packets. Default is just the first 64
-X == dump the packet as ASCII not just numbers
The rest is the filter. We want packets that are to/from the backend host and on the port you're talking to.