The method I use is to check the clearing time.
For this test you want use that little piece of film leader you cut before winding the film onto the developer reel. Start a timer and dip the film leader into a solution of fresh, never used fixer and leave it there. Keep watching and pull it out when the film becomes transparent. It may not be obvious as the transition is gradual.
Record the time it takes to ‘clear’ the film leader in this fresh fixer. As you reuse the fixer and test before future use, when it takes twice as long as this initial test it is time the fixer is no longer useable.
A rough guide for minimum film fixing times is twice the tested clearing time plus one minute. Example if your film leader clears in one minute, then absolute minimum fixer time for you negatives would be 3 minutes. I would never go with a fix time that short no matter how fast the test strip clears.
I usually fix for 5 minutes. I have always heard that a too short fix time can cause your negatives to degrade over time. I would rather go a little long than go too short.
Another trick I use is to keep a large piece of white tape on the jug my fixer is stored in. I record each date and time I use the fixer.
I use Ilford Rapid Fixer and the general wisdom says a liter of fixer mixed 1+4 should develop 20 or more rolls of 35mm film. I usually dump mine at 10 or so, or when the test strip clearing time has doubled. Fixer is cheap so I am not going to risk my negatives by trying to save a few pennies on fixer.
I hope this helps. As always thanks for stopping by.