What's the best way of checking fixer (for prints.)?
For film I use the clearing time as a guide, needless to say that can't be done by watching paper. So how do you do it, based on number of prints, area of paper, do you do a film check, or is there another method?
I use a water-wet test print and blot one corner dry, apply a few drops of KRST (1:9) and wait 2-3 minutes and blot it off.
If there is ANY stain either the fixer is shot or fixing was not complete.
Tetenal sell a so-called fixer bath test (test strips)
A tad pricey but they work ok.
I do a film clip test. I know that, for example, the TF-4 fixer that I use will, when fresh, clear a piece of film in about 30 seconds. When that time creeps up to 45-60 seconds, I take no chances and dump the fixer. Does it still have some life left at this point? Probably, but given that fixer is inexpensive, I prefer to play it safe.
If you shoot and process your own 35mm film you can save the clipped leaders and use them as little test strips. See how long it takes for a piece to clear in fresh fixer and when that time gets close to doubling, it's time to dump the fixer. You might even consider it worth your while to sacrifice a roll of the cheapest B&W film you can find just for this purpose. For $2 to $3 US you can have a slew of test strips, since you need a very small piece to perform the test. I've found that if you soak the film in some water prior to the test, you'll get a more accurate result. This has the effect of swelling the gelatin somewhat so that the fixer can penetrate the emulsion more quickly.
I believe you meant "How long should you fix for," right?
I don't have an answer, but though that maybe your question was missunderstood.
these might help:
look at the "residual silver test" and "residual hypo test"
These should help you determine how long to fix and wash for. I haven't tried them yet, but I might, someday....
If this is not what you meant, and you just need to know how to test for fixer exhaustion, Photog Formulary has a product for that as well, although I use the "Edwal" equivalent (tests for silver content of fixer)
Hope this helps