Can I find out if an airline has changed the flight time after my customer has already booked?

Yes, in most cases you can see schedule changes on the booking itself, though how fast you hear about them depends on the airline and on the supplier the ticket was issued through. When a carrier moves a departure, cancels a leg or changes an aircraft, that update is normally pushed back into the booking record, so when your staff open the booking they see the revised timing along with a note that the itinerary has been changed by the airline.

The honest limit is timing. Airlines send these notices at their own pace, so one change may arrive many weeks before departure and another only a day or two before. Some low cost carriers and some content sources do not send structured change messages at all and will instead email whichever address is on the ticket, which means your portal may not know about it until someone checks. For that reason the system should be your main record of what has happened, but not your only safety net for flights leaving very soon.

What you do next also varies. Many setups can alert your operations team by email or flag the affected booking so someone picks it up, but accepting a new time, moving the passenger to another flight or claiming a refund usually follows the airline's own rules, and for some suppliers it has to go through their support desk rather than an automatic request.

It is worth asking which of the airlines in your content send change notices automatically, confirming whose contact details sit on the tickets you issue, and keeping a simple daily habit of reviewing departures in the next few days.