This is a very quick guide to testing your film fixer. Unless you make fresh fixer every-time you develop your film you might be using your fixer more than you really should. I have in the past pushed mine a little too far and have regretted ending up with poor negatives that required to be re-fixed. So now if I have any concern at all I test my fix or just make a new batch.
1. Retain the leaders that you are cutting from your film.
2. Pour some fixer into a container
3. Put the leader in the fix
4. Agitate the mixture and time how long it takes for the film to go clear
5. This is for how long you should fix your film .
Any more than 3 mins and I discard the fixer and make a fresh batch.