Take some of the same film that you are using. What you want is a sample of exposed leader. Of course, being C-41, it should be any color neg, but the universe is perverse.

Plop it in a bit of the fixer in question, and time the, um, time until clear. Double or triple that to be safe.

You may find these old posts of mine pertinent. Or, not.

http://www.apug.org/forums/forum226/...ter-fixer.html

http://www.apug.org/forums/attachmen...rative.pdf.att