In critical code we have a lot of things like

   if seek(bintoc(m.nMonat) + bintoc(m.nBeleg), [z_Image])
      assert z_Image.MONAT == m.nMonat and z_Image.BELEG == m.nBeleg
      ...

because the test is affordable and so it would be unwise not to do it. However, tests like these can only uncover cases where an index finds wrong things; they cannot detect other modes of failure like positioning on the wrong record or failing to find a key that is actually present.

So, in the absence of SET INTEGRITY ON we revert to the Turandot principle. I.e., if the question is "How's the index doing today?" we answer (FX: tappety-tap, DELE TAG ALL, INDE ON ...) "Fine, just fine!". *g*

Also, for some critical stuff we force Fox to skip indexes by using expressions that break Rushmore, like FOO+0 etc.