> FileTest.exists?("filename")

Aha!   I didn't look closely enough.  I see it now, listed under
"modules" in the built-ins chapter.

I would probably have noticed it had I been looking at the paper version
of the book, but I was sitting in front of the PC, so I just checked the HTML.

Obviously, I should make myself a little more familiar with the built-in
modules !!

Thanks David.