How to correctly test for existence of pseudo tty?

I'm trying to debug some scripts that are supposed to run at boot time
through an rc script in /sbin/init.d and I suspect there's an issue
related to the lack of an associated tty.

The scripts work fine if I run them interactively. In particular, its
a perl script that fails.

A calling shell script does an su to an account that assumes ownership
of the process. The account uses ksh and in its .profile it has this
snippet of code -

if tty -s; then
....
fi

To my surprise, when I ran the process from cron, I got this output in
a temporary log file -

ttytype: couldn't open /dev/tty for reading
stty: : Not a typewriter
Not a terminal
logout

It turns out that cron attaches a pty to the process it runs (unlike
init). Obviously, the above output was due to there being no attached
terminal.

So, how can I correctly check in the .profile script if there is an
attached terminal? That is, whether the login is interactive or not.
'tty -s' and 'pty -s' don't appear to do the right thing and I can't
find another command that checks this correctly.

.

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