It seems like many of us are using fixer rather sloppily, with no real check on activity before using... The clearing test is simple and should be standard practice for determining fixing times and exhaustion for film (paper is another issue; use throughput as a guide and test for residual silver; see below).
Test for film: Place a drop of fix on the film, wait 30 seconds, then immerse in fixer with agitation and time how long it takes for the film to clear and the spot where the drop was to be indistinguishable from the surrounding. Do this with fresh fixer to establish a baseline. Then, test your fixer when necessary, i.e., if you even vaguely suspect it might be exhausted for whatever reason. Toss the fix and mix new when the clearing time in the old fix is double that of your baseline test. Use the same film for testing, of course! Best case is if you test with the film you are developing. EZPZ and accurate. The test detects both fixer that loses activity from both age and overuse. Any fixer that has precipitate floating around in it, or has become cloudy shouldn't be used, especially for film where the particles can adhere to the emulsion.
A word about fixing times: The rule of thumb is to fix for at least twice the clearing time. However, don't just use your baseline clearing time for every batch! As the fixer exhausts, but before it needs discarding, fixing times will increase. You can do a clearing test before each batch to determine the proper clearing time for the particular state of exhaustion your fixer is in, or you can simplify the process and establish one, longer fixing time that is at least four times the time it takes to clear in fresh fixer. Yes, this is longer than necessary for fresh fixer, but just right for fixer that is just about to be discarded. Since fixing film this much longer does no harm, I find it a lot easier to just use the longer time and not worry about testing for fixing time every batch. I toss my fix before the clearing time reaches 2x my baseline time, so I'm always safe. Plus, the extra time in the fix helps with removing the pink stain many films have.
For paper: Throughput can be used as a guide to capacity for papers. RC paper is easier to fix than fiber-base and can be fixed with a one-bath regime. For fiber-base paper and processing for optimum permanence, one-bath fixing is certainly possible, but the capacity of the fixer is limited to about 10 8x10-inch prints per liter. Two-bath fixing is more economical. A simple test like the film-clearing test isn't available for papers. The only way to be relatively confident that you are not using your fixer beyond its capacity is to test for residual silver using the Kodak ST-1 test or by using Kodak Rapid Selenium Toner 1+9 (my preference). There's a ton of information about the tests on the site here that a search will turn up. Here are the basics: A drop of ST-1 ore KRST on the print border, wait 3 minutes, rinse and see if there is a discoloration. Anything darker than a barely-perceptible cream color means that there is residual silver and the fixation was inadequate, i.e., too many prints through the fixer. Find the correct throughput for your regime, add a safety factor and you're good to go. Once you've established a workflow that gives you clean prints you can test occasionally to make sure nothing's changed; I test the last print through the fixer before discarding and mixing fresh as a habit.
Best,
Doremus