Usually I used TCPdump from shell to capture the the life packet on Junos. Yesterday, I’ve got another TCPdump command from my friend blog, Przemyslaw (Thanks dude). The good thing is, now this command available from cli 
Here it is:
aragorn@gandalf> monitor traffic interface ge-7/0/0 write-file debug-bgp.cap
You can use wireshark to open the captured file. However, this command is hidden command, so have to type it completely.
