Lots of long answers here but I have found that the retained Silver test and the retained hypo test are the best. The Formulary uses Kodak formulas for making them up. The exhausted fixer test is virtually useless especially when using papers. Testing for 2x clearing time in film is quite useful as well.

I spent years on the subject of fixers and find the subject fascinating.

Oh, the Formulary sells a full line of film and paper solutions for development, stop, fixing and post wash testing. I have never used a hypo eliminator or any such was aid. They are virtually useless and increase the pollution factor.