Castle upkeep can be paid during the last 3 days before it expires, the leader, officers, governor, and banker should be able to pay this. They should receive a mail 24 hours before the upkeep lapses.
All members of the team will receive this message and not just the officers and leader so as to avoid any issue
Since version 4.99, a new feature has been added which is "Auto Upkeep". This automatically pays the upkeep of the castle once it has 24 hours left. If it fails to pay due to insufficient gold, it will do it again within the last 2 hours before it is due. If it still fails to pay, the team who owns the castle will have to manually pay it.
Once we have the name of the Castle, we can load the Castle in PX2 > Castle Browser, to see when the upkeep is due, which gives us an idea about the date of the last payment.
Also, in the player's Atlas Webview, we can load the Castle's management interface, then select the Bank's Ledger, to find the last upkeep payment, which will be listed as "Upkeep".
We have a tool which we can use to verify if an upkeep has been paid which is the "Atlas Castle Infrastructure Logging History". This will display all information that took place on the castle. Simply click the home icon on the upper left of PX 2 and then click the "Atlas Castle Infrastructure Logging History" and place the castle ID and it will populate all logs related to the castle.