[racket] Check for #<undefined>

I have a structure with a guard function. It works well.

However, I want to use it in a (shared ...) construct, and when the
guard runs, a value is #<undefined>. I'm fine with letting the guard
accept #<undefined> as a legal value, but I can't figure out any way
to test for it.

Is there such a beast, or is #<undefined> untouchable?

Todd