If it is a neutral or alkaline fix, and depending on formula, this may not apply. These fixes will feel somewhat slippery too. There is no definitive test for these as there are too many possible formulas.
Haaa, I keep forgetting about alkaline fix... Depends on the formula of course.
Are you using ID 11 at stock dilution, or are you diluting it 1 + 1?
I ask, because if for some reason you "developed" the blank films in just the diluting water, the result would have been the same as the result obtained by fixing the film before developing it.
“Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”
Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2