Normally new fares are filed during business hours on weekdays, and typically have expiry dates published with the fare, and/or sell-by or travel-by dates in the fare rules text. However, someone from the UA fare department can publish or revoke a fare at any time.

Anyone flying a routing including that sector which has a valid G fare published for it.

If you have a route, people with GDS subscriptions can look, although historical granularity is usually daily, so if it was a mistake fare or something like that it wouldn't leave a trace.