Let's be honest, who routinely tests their fixer solution anyway? I certainly don't. I have tested prints for residual fixer in the past, and as a result my routine now errs on the side of extravagance rather than risk stains on my prints.
More than you probably wanna know, but an example of a serious processing lab, pretty large, operating under a stringent sewering permit, vs "amateur" processing...
Speaking from my long-ago lab background, we "tested" every new mix of C-41 fixer via a pH and specific gravity check. This doesn't actually test for "usability," but rather checks that the mix operator has not made a significant error.
Then, each morning we would have the processor operators pull samples from each of their chemical tanks. We'd do the same checks on each fixer tank, pH and specific gravity. (On our C-41 machines we ran 3 fixer tanks, counter-current flow replenishment, squeegees between each tank. This was an effluent control measure, to drastically reduce silver going into the wash water.) The specific gravity checks were an indirect way of confirming that the squeegees were functioning correctly - each tank runs in a different range. It's also a cross check on the replenishment rate.
Then, during the processing day, we'd run a control strip every two hours on each C-41 processor. The first one was before the machine was cleared to start up on production work, and a final one at machine shutdown. These strips have test patches to screen for fixer problems, so each control strip sorta doubles as a crude fixer test.
When the machines were initially set up, and periodically after that, we'd run silver analysis on each fixer tank. This would confirm proper operation of this part of the "effluent-control system."
There's more. Overflow from the first fixer tank goes into a collection system for silver recovery. This was a manually-run batch system using electrolytic silver recovery. We wanted to push it as low as possible, without "sulfiding," prior to "tailing." Because this is a fairly sensitive end-point, right on the verge of sulfiding the cathode, the operator would pull a sample for silver screening. Our chem lab would do a 2-minute test, then give the operator a time, in minutes (at reduced amperage) to finish up the cycle. (On completion that fixer gets sent to a tailing system.)
On occasion, especially if there was a problem with the electrolytic silver recovery, we'd run a set of chemical analysis at intervals throughout the process. These would include thiosulfate, sulfite, and silver content, all referenced against the total amp-hours of plating current.
That's about on our fixer testing.