I'm not sure I understand. My point was that you could test whether the fixer was working; if you haven't fixed the film enough, yes, you can refix; but it's pointless if the problem is with the fixer itself.
The rule of thumb I was given years ago (and it is slightly out of date, but worth following) was to fix for twice the time the film took to clear, and when the clearing time had doubled, throw the fixer. Some modern films require more than the double time, but as far as I know the "doubled clearing time = throw fixer" still holds. I use some end of a bulk roll FP3 (sic) to test my fixer.