If you shoot and process your own 35mm film you can save the clipped leaders and use them as little test strips. See how long it takes for a piece to clear in fresh fixer and when that time gets close to doubling, it's time to dump the fixer. You might even consider it worth your while to sacrifice a roll of the cheapest B&W film you can find just for this purpose. For $2 to $3 US you can have a slew of test strips, since you need a very small piece to perform the test. I've found that if you soak the film in some water prior to the test, you'll get a more accurate result. This has the effect of swelling the gelatin somewhat so that the fixer can penetrate the emulsion more quickly.