Edwal sells a made up iodide solution called Hypo Check.
  The formula given above and instructions are in the
_Kodak Black and White Darkroom Dataguide_ which is not
expensive and should be available from any of the larger
photo suppliers that still sell darkroom supplies.
  Another way to test fixer is to fix out a scrap of film
in the fresh fixer and see how long it takes to clear. Then
test the fixer periodically with scraps of the same type of
film to see how much longer it is taking. The old rule of
thumb is to discard the fixer when clearing time has doubled
but this is probably too far exhausted.
  Two bath fixing very much extends the life of fixing
baths because the second bath can clean up after a first
bath this is too exhausted to be used on its own. When the
first bath reaches its capacity the second bath is
substituted for it and a fresh second bath mixed up. This
routine extends the life of fixer, especially for archival
purposes, for from four to ten times that of a single bath.
The use of a sulfite wash aid, like Kodak Hypo Clearing
Agent, after fixing further extends fixer life because it
allows the washing out of some otherwise incompletely
soluble silver complexes.
  The problem with re-using a wetting agent bath is that it
can eventually become contaminated with residue from the
film, either gelatin or hypo. Gelatin will allow growth of
bacteria in the bath. Photo Flo is cheap enough to use
one-shot.
  A good final rinse for film is:
Water to make 1.0 liter
Isopropyl alcohol, 99%   25 ml or if 70% 35ml
Photo Flo  2.5 ml (half the usually recommended strength)
Soak the film in this for about 2 minutes after washing and
hang up to dry without squeegeeing. This should be discarded
after use.
  If rubbing alcohol is used make sure its Isopropyl
alcohol and that its only alcohol and water, not the kind
with Oil of Wintergreen or some other flavoring in it.

Used to run a sink line for black and white, 3.5 gallon tanks.  We reused
photo-flo, with tanks that deep we didn't get much crud on film unless we
agitated the solution when we put the hanger rack in but let me tell you the
mold, algae and bacteria that can grow in a tank of photo-flo are really
nasty and have a habit of depositing only in the nearly clear areas on film
where they will subsequently stick forever.  Stinks when you clean the
tanks, too.

And why bother.  I used wetting agents at extreme dilution's, one or two
drops per roll.  I am still using from bottles my chums bought for a film
class in 1976 and then gave to me when they finished the class.

Test solutions for fixer are a bad idea,  they only tell you, post mortem,
that your fixer is already dead,  much better to use two baths and discard
the first fixer after 100 8x10 prints per gallon (about 1/2 of Kodak's
recommendations).  Then fixer 2 becomes fixer 1 for another 100 prints.
Currently $12.00USD will fix 100 prints, that's 12 cents per print.

For film, I process one shot, using a Unicolor film base, my fixer usage is
pretty low, I use Kodafix at paper dilution, one shot for film.  I have been
known to reuse this fix for RC contact prints.

I would suggest storing exhausted fixer until you have a sizeable amount
- say a gallon or so. There are companies who remove the silver for
re-cycling - this is much better than simply allowing it to be discarded
in a landfill.