What Gerald Koch said: If the fixer is still clear with no particles floating around in it and it has a slightly acidic smell, but no smell of "rotten eggs," then mix up a batch and do a clip test. If it clears film in 45 seconds or less, it's still just fine; use as normal.
Expiration dates are based on a worst-case scenario of storage and handling. They should really read, "Good until at least...". Fixer that "expired" this month is most likely still good as new. It's not Mission Impossible: "This fixer will self-destruct in five seconds."
Best,
Doremus