How can I know if the other person is on or off without using the mobile phone?

I don't know what model your mobile phone is.

Ordinary mobile phones have SMS receipts.

Will tell you that the message has been sent to the other party.

Some are turned off by default.

Your mobile phone, if you have it.

You can turn on this function.

Send messages to each other.

If there is a receipt.

It means that the other party's mobile phone is on.

In other words, you can receive it

If the other party shuts down

No receipt.

When the other party's mobile phone is turned on,

The text message will be sent to the other party's mobile phone.

At this time, the receipt information will be sent to your mobile phone again.

You will know when the other party turns on the phone.