Depending on the device type, you can run a few things. If it is an SSG, then you can use the following.
set pps. -- Turns on packets per second counting
get pps -- Displays information
You can also do a packet profile that will show what is sending the majority of the traffic.
set fprofile packet en
set fprofile vector en
set fprofile packet start
# <wait for a few seconds>
set fprofile packet stop
get fprofile packet
get fprofile packet ip
get fprofile packet ip dport
get fprofile packet ip dst-ip
get fprofile packet ip sport
get fprofile packet ip src-ip
get fprofile packet ip proto