With film you can reasonably do the twice the clearing time method.
Test with a scrap or piece of leader before processing, or else, check the film after a minute in the fix, then perhaps every 30 seconds afterword. Note how long the test strip or rebate takes to be completely clear, then extend the time by how long that took. You'll probably be at 3 to 4 minutes total in fresh fix, somewhat longer as it's used.
The real method would be to use a residual silver test which is mentioned in the link.