Say you wanted to see the dialplan for extension 23 in the context [my-phones]; you would do this with asterisk -rx "dialplan show 23@my-phones" entered in the shell:

root@molokai:~>asterisk -rx "dialplan show 23@my-phones"
[ Context 'my-phones' created by 'pbx_config' ]
  '23' =>           1. Wait(1)                                    [pbx_c
onfig]
                    2. Answer()                                   [pbx_c
onfig]
                    3. Playback(hello-world)                      [pbx_c
onfig]
                    4. Wait(1)                                    [pbx_c
onfig]
                    5. Hangup()                                   [pbx_c
onfig]

-= 1 extension (5 priorities) in 1 context. =-
root@molokai:~>