Um... If you are talking about users on a network of linux machines, and determining what shell gets invoked when people login or otherwise connect, wouldn't you just look at the entry for the given user in /etc/passwd? That is what sets the login shell.

If it's a matter of warning users about the risks starting up other shells after they are logged in/connected, that should be in the text of "/etc/motd" ("message-of-the-day"), which gets displayed at login (before the initial shell prompt). But as for controlling shell behavior at login, /etc/passwd handles that.

(Or maybe I'm completely missing the point about your question...)