i had a conversation with the good folks at sprint about this years ago, mainly because the hypo check is readily available
and seems like it would be a reliable test. i was told a few things ... first, it is unreliable with some fixers.
some have a larger capacity than edwal's fixer and , some don't. AND most people don't use the hypo test drops correctly.
they tilt the tray or beaker and put a few drops in and see if it gets cloudy ( and that's it )
... the instructions on the bottle
used to suggest to do it differently and this yields different results than a few drops in a tray &c
it doesn't say it on the enlarged photograph above this post, but it USED TO SAY remove an ounce of the fixer in question,
you put a 2drops of hypo check in, it may cloud up, but you mix
the 1oz and see if it clears. if it clears the fixer might still have some life in it, if it stays cloudy it toss it.
maybe the posted image is a newer bottle with different instructions of "edwal's hypo check" and the instruciton on the bottle are different ..
aside from the test strips ralph mentions, the good folks that make the silver magnet also have test strips to show how much silver is in the fixer ( and i sell them ). easy to use ( like litmus paper ) .
that said, during the phone conversation i alluded to earlier, the folks at sprint suggested to use the clip test with their fixer.
( mattking's article on the clip test here: (there was a url link here which no longer exists) )
it is an age old test that they believe is a good indicator of whether fixer is exhausted or still has life in it.
they also have a SYSTEM ( its the system in sprint system of photography ) where if you use their developer, stop, fix and fix remover .
the WHOLE SYSTEM is exhausted when their stop bath indicates it is spent. i used to use their entire line and not worry about
hypo check and what was exhaused and what wasn't because the stop indicated and it was ez.
whatever test you use, good luck !
john