You must remember that it is almost impossible to check a tank for Silver content. This is especially true when using the fix for paper.

The most reliable test is "time to clear". Select a 35mm film type (whether you are using paper or not) and cut an unexposed roll into 2" pieces. Using fresh fix, determine the clearing time. Now, every time you make a run, do the same with another piece of film. When the clearing time is approaching 2x the original time, that fix (first or second "tank, tray or whatever") is shot.