I use a hypo check, when using kodak fixer. A few drops to see if cloudy precipitate forms and stays.

At home I use ilford rapid fix and now currently hypam. I mark up to 24 rolls on 1L seltzer bottles.

I use a 2 fix setup, after the 1st bottle has had 24rolls in it, I dump and substitute with 2nd bottle. Then mix a fresh batch for bottle 2 again. I fix films for 3min each or 6 total in the rapid fix.

This gives me a wide and safe margin, without over fixing. I think the rapid fix can do more than 24rolls, plus sometimes I shoot 24exp or 12exp test rolls and still mark as if they were all 36exp.

Has worked well for me. If in doubt just make fresh fixer, shame to mess up film you take the time to shoot on something as cheap as fixer.

Hypam is like $25 for that huge 5L jug. Which you dilute down further. Kodak fix is like $6 for the gallon size. How much does your film cost? No brainer.