Start ngrep either on the Asterisk server or on the client:

ngrep -s 1500 port 5038 -T

A successful login results in output like this:

interface: eth1 (10.13.0.0/255.255.255.0)
filter: (ip or ip6) and ( port 5038 )
####
T +7.653003 10.13.0.102:5038 -> 10.13.0.55:46779 [AP]
  Asterisk Call Manager/1.2..
##
T +0.113468 10.13.0.55:46779 -> 10.13.0.102:5038 [AP]
  action: Challenge..actionid: 29181730_0#..authtype: MD5....
##
T +0.000391 10.13.0.102:5038 -> 10.13.0.55:46779 [AP]
  Response: Success..ActionID: 29181730_0#..Challenge: 171465739....
##
T +0.036776 10.13.0.55:46779 -> 10.13.0.102:5038 [AP]
  action: Login..actionid: 29181730_1#..key: fdc46063e81c6c5ab84188698e0cb55b..authtype: MD5..username: manager....
#
T +0.001271 10.13.0.102:5038 -> 10.13.0.55:46779 [AP]
  Response: Success..
#