How can a mobile phone send a message to the other party to know that the other party has seen it?

In this case, you can consider handling the "SMS receipt" business.

The short message receipt service is a short message value-added service launched by China Mobile for individual users. After using the mobile phone to send SMS normally, the mobile phone will immediately display the words "Message sent". But this only shows that the short message has reached the short message center normally; Doesn't mean the other party received it.

After handling the business, the other party successfully receives the short message, and the sender will also receive the short message prompt.