You can test fix time without risking exposing the film to light. Snip off the 35mm leader, or a narrow piece of the end of a roll of 120 film, and test it in the light.

Put a drop of fixer on the film. Wait about fifteen seconds. Drop the film into the fixer. Watch. When you can no longer see the spot where fixer got a head start, that's your clearing time. Double that for non-T-grain films, and triple it for T-grain films. If your fix time has doubled from what it was with fresh fixer and that film type, discard it and mix anew.