11.3 sysutils/jailer

This utility must be installed inside the jail and has the ability to shutdown/restart it.

I haven't used this utility because calling /etc/rc.d/jail start/stop does the same thing. Besides ..jailer creates its own process inside the jail and if an attacker gains nobody's privileges he is able to list the processes and see that he's actually into a jail.

Anyway, the injail command in this package looks interesting. It determines if a process is running inside a jail and might be used for scripting purposes.

Another way to determine if a process is running in jail is to parse the output of ps(1)