On Linux there is a nice tool for testing connections, bandwidth, opening temp. connections etc:
What is equivalent on Juniper firewalls?
get route , get tcp , it depends on what you want to accomplish .
Please have a look here http://fir3net.com/Netscreen/juniper-commands.html and here http://www.juniper.net/techpubs/en_US/junos/information-products/topic-collections/swcmdref-basics-services/swcmdref-basics-services.pdf
(I know this is a ScreenOS question but...)
In current JunOS nc isn't there, but you can just use telnet to arbitary ports.
If you need incoming you may have some luck using inetd.