How does iphone5s determine that the SMS has been sent? Will it be displayed if the transmission is unsuccessful? Because I'm not sure whether the mobile phone was in arrears at that time, although

If the other party is also an iphone, the SMS will be sent in the form of iMessage (using data traffic, no SMS fee). When the other party receives it, the sender will prompt that it has been sent. If the other party does not receive the MMS, it will prompt that the sending failed.

If it is a short message, it can be received if the other party is not turned on or out of service within 48 hours. Sending a text message will not prompt whether it is received or not.

Only the information sent through iMessage can see whether it was sent successfully. Ordinary SMS/MMS has been sent to the SMS server of the network operator without a "undelivered" prompt (whether the other party is turned on or not, it is impossible to confirm whether it has been received).

note:

IMessage is an instant messaging software launched by Apple, which can send short messages, videos and so on. It has high security. IMessage uses the new message reminder system of iOS and Mac os to directly push information to the other party's screen, regardless of whether the other party is in the game or locked. If both parties are using iMessage, you can even see the state of each other's speech.