You can check it from System Monitor Log of each Mirror member. You can see an example of the log:

05/22/23-07:22:26 [SYSTEM MONITOR] System Monitor started in %SYS
05/22/23-07:26:07 [SYSTEM MONITOR] Mirror status changed. Member type = Failover, Status = Backup
05/22/23-07:33:01 [SYSTEM MONITOR] System Monitor started in %SYS
05/22/23-07:33:01 [SYSTEM MONITOR] Mirror state: Member type = Failover, Status = Synchronizing
05/22/23-07:33:31 [SYSTEM MONITOR] Mirror status changed. Member type = Failover, Status = Backup
05/22/23-07:35:55 [SYSTEM MONITOR] System Monitor started in %SYS
05/22/23-07:35:56 [SYSTEM MONITOR] Mirror state: Member type = Failover, Status = Transition
05/22/23-07:36:26 [SYSTEM MONITOR] Mirror status changed. Member type = Failover, Status = Waiting
05/22/23-07:36:26 1 alerts posted in messages.log
05/22/23-07:40:26 [SYSTEM MONITOR] Mirror status changed. Member type = Failover, Status = Backup
05/22/23-07:58:15 [SYSTEM MONITOR] System Monitor started in %SYS
05/22/23-07:58:15 [SYSTEM MONITOR] Mirror state: Member type = Failover, Status = Waiting
05/22/23-08:06:43 [SYSTEM MONITOR] Mirror status changed. Member type = Failover, Status = Backup
05/22/23-08:10:37 [SYSTEM MONITOR] System Monitor started in %SYS
05/22/23-08:10:37 [SYSTEM MONITOR] Mirror state: Member type = Failover, Status = Waiting
05/22/23-08:34:40 1 alerts posted in messages.log
05/22/23-08:35:10 [SYSTEM MONITOR] Mirror status changed. Member type = Failover, Status = Backup
05/22/23-10:50:25 [SYSTEM MONITOR] System Monitor started in %SYS
05/22/23-10:50:25 [SYSTEM MONITOR] Mirror state: Member type = Failover, Status = Waiting
05/22/23-11:00:15 [SYSTEM MONITOR] Mirror status changed. Member type = Failover, Status = Backup
05/26/23-11:03:06 [SYSTEM MONITOR] System Monitor started in %SYS
05/26/23-11:03:07 [SYSTEM MONITOR] Mirror state: Member type = Failover, Status = Transition
05/26/23-11:03:37 [SYSTEM MONITOR] Mirror status changed. Member type = Failover, Status = Backup
05/26/23-11:05:08 [SYSTEM MONITOR] Mirror status changed. Member type = Failover, Status = Primary
05/26/23-11:05:10 1 alerts posted in messages.log

I usually use backup and restore functionalities, the journal is synchronized automatically. You can see it in this article:

https://community.intersystems.com/post/recovering-mirror-after-crash

You know, I'm a superstar! 😁