The other way for judging fixer exhaustion fixer, is how much silver it has had to dissolve in the course of it being used. Imagine you shot all rolls of pure white scenes. Then the negs would be all black. The only undeveloped silver would be from between the perfs if 35mm. This is a lot differnent fixing load than someone who is shooting candles in a coal mine, with almost clear negs as a result. So the exhaustion point on fixer will vary depending on the subject matter.
I mix fresh fixer, and drop a drip of it onto a film leader, wait 30 seconds or so, then drop the film leader (the part I clip from the start of a 35mm film when loading on the reel) into a small clear glass of fixer. Time how long until the whole of the leader is as transparent as the spot that got the early drop, and write it on the storage bottle for the fixer. I also use this first test to tell me how long to fix; it is twice this clearing time, although I usually fix approaching 2.5x, to the next nearest minute. Don't radically overfix with a rapid fixer; some can actually dissolve away fine details.
Then when you are getting close to the rated capaicty of the fixer (when known), repeat the leader clearing test. Repeat more frequently if you don't know the capacity.
When the clearing time is twice the original fresh clearing time, I consider the fixer at capacity of it's ability to dissolve more silver efficiently. It also is a good guide of when it is increasingly difficult to wash out of the media; not so much a deal with film, but certainly with fibre based paper. By the way, I do the same fixer test with my fixer I use with paper.
I also use the silver nitrate hypo test solution, but I find that it almost always indicates that the fixer has longer to go than my doubling of fresh time testing.