Hi,
If you want to see transit traffic, firewall filter can be used and log can be captured like below. For the live traffic to and from the device "monitor traffic interface extensive" can be used on the desired interface.
https://www.juniper.net/documentation/en_US/junos/topics/reference/command-summary/show-firewall-log.html
user@host> show firewall log extensive
Time of Log: 2016-01-17 22:16:21 PST, Filter: pfe, Filter action: accept, Name of interface: xe-0/0/1.0
Name of protocol: UDP, Packet Length: 98, Source address: 203.0.113.1, Destination address: 203.0.113.1
: 00-0F: 00 01 03 ee ee ff 00 01 - 09 22 55 ee 81 00 02 58
: 10-1F: 08 00 45 00 00 62 00 00 - 00 00 40 11 77 8a 01 00